microwave sensors – active and passive - byu mers laboratory

Chapter 6

Microwave Sensors – Active andPassive

David G. LongElectrical and Computer Engineering DepartmentBrigham Young University Center for Remote Sensing459 Clyde Building, Provo, UT 84602, [email protected]

Revision Date: January 28, 2008

SummaryMicrowave remote sensing instruments provide another view of the environmentwhich can augment traditional visible and infrared observations. Microwave sen-sors can be classified as either passive (radiometers) or active (radars). Each sensorclass provides unique insight into the electrical and mechanical properties of thesensed environment. This chapter provides a very brief overview of microwave re-mote sensing along with summaries of the capabilities of major microwave sensors,particularly space-based sensors.

6.1 IntroductionThe electromagnetic spectrum extends from low frequency radio waves up throughhigh energy gamma waves. Microwaves extend over an important part of the elec-tromagnetic spectrum. Typically microwaves are defined as frequencies from ap-proximately 1 GHz to 300 GHz (Fig. 6.1), corresponding to an electromagnetic

1

2 Manual of Remote Sensing Vol. 3

wavelength range of 1 m to 1 mm (Ulaby et al., 1981). At the low end of thisrange, the Earth’s atmosphere is transparent, but becomes more opaque with in-creasing frequencies. Over the range of microwave frequencies, the radiative emis-sion and absorption of the atmosphere is sensitive to geophysical parameters suchas moisture, rainfall, and temperature. Thus, microwaves play an important role inremote sensing of the atmosphere as well as the Earth’s surface. Microwaves arealso widely used in astronomy to study cosmic phenomena.

1 3 30 100

BandL S K Q VC X WP

0.39 1.55 3.9 5.75 10.9 36 46 56

Frequency (GHz)

Wavelength (cm)

100.3

100 30 10 3 1 0.3

300

0.1

Figure 6.1: Illustration of the microwave region of the electromagnetic spectrumwith common band definitions shown. The precise band definitions vary amongauthors.

Microwave sensors have been flown on aircraft and spacecraft and have be-come standard instruments for operational observations of the Earth. Extensivedatabases of observations made by various types of sensors at different frequencieshave been collected of the Earth, the outer planets, and beyond. This chapter brieflyconsiders the theory and methods of microwave remote sensing. While the focusis on airborne and spaceborne observations of the Earth, other sensors and targetsare considered. The primary goal of this chapter is to provide some sensor-specificinformation on past, present, and future microwave sensors and summarize a fewkey analysis techniques and applications. Major sensors and/or sensor technolo-gies are briefly described, though only non-military sensors which have provideddata sets available to the general remote sensing community are included.

After this introduction, the next section focuses on passive microwave sensors.The succeeding section discusses radar systems. In each section the general theoryof operation of the sensor class is provided, followed by a very brief description ofselected sensors. Further information is available in the cited literature. A generalintroduction to microwave remote sensing concludes this section.

6.1. INTRODUCTION 3

6.1.1 Passive versus active microwave sensing

Microwave remote sensing instruments can be divided into two broad classes: pas-sive, known as radiometers, and active, known as radars, see Fig. 6.2 (Skou andLe Vine, 2006; Ulaby et al., 1981; Woodhouse, 2006). The latter from radiometersin that they include a transmitter. Both classes of sensors have been used on aircraftand spacecraft to study the Earth and the other planets.

Microwave Sensors

Passive (Radiometers)Active (Radars)

Real Aperture Synthetic Aperture (SAR)

Scatterometers

Altimeters

Weather Radars

Real Aperture Synthetic Aperture (Interferometers)

Figure 6.2: Organization of the major classes of microwave remote sensors.

Active microwave sensors can be further divided into four general classes: syn-thetic aperture radar (SAR) systems, scatterometers, altimeters, and weather radars.The latter systems typically use real-aperture antennas while SAR is based on syn-thetic aperture antenna processing techniques. Some hybrid systems have beendeveloped. Inverse synthetic aperture radar has been used for ground-based sens-ing of extra-terrestrial bodies.

SAR systems are radars designed to make high resolution radar images. Theyoperate by transmitting modulated pulses and using Doppler/range processing toconstruct backscatter images. Typically, they are not as well calibrated as scat-terometers. Scatterometers are designed to measure radar backscatter very pre-cisely, but typically have lower resolution than SARs. They tend to be less compli-cated than SARs. Altimeters are radars designed to measure height or distance,though other information such as radar backscatter is extracted from the echo.Weather radars are specially designed scatterometers which have ranging capa-bility. They are designed to measure rainfall and other meteorological phenomena.

Passive microwave sensors rely on the thermal emission of microwave signalsfrom objects. The emission is related to the physical temperature and electricalproperties of the sensed surface (Fung, 1994), with modulation by the interveningatmosphere (Ulaby et al., 1981). Since they contain no transmitter, passive sensorstypically require less power to operate. To reduce the noise level of the measure-


ments they typically operate over a broader frequency range (band) than do radarsensors, which tend to be narrow band. While most radiometers use real-apertureantennas, radiometer-interferometers often rely on multiple antennas (Goodberlet,2000; Ruf et al., 1988; Wohlleben et al., 1991).

6.1.2 Airborne Versus Spaceborne SensingThe first microwave sensors were ground-based, but aircraft-based applicationsquickly followed. Microwave radar systems were first deployed on aircraft inWorld War II and microwave sensors flew aboard spacecraft early in the spaceage. Ground-based microwave sensors have been widely used in astronomy andweather.

Currently, some of the most important Earth sensing microwave instrumentsare based on spacecraft. Spacecraft basing offers a number of advantages overaircraft-basing, including much broader spatial (often global) and temporal cover-age. However, operational costs are much higher for orbiting sensors than flyingthem on aircraft. Government agencies such as the U.S. National Oceanic andAtmospheric Administration (NOAA), the U.S. National Aeronautics and SpaceAdministration (NASA), the Russian Space Agency (RKA), the European SpaceAgency (ESA), and the Japan Aerospace Exploration Agency (JAXA) operate afleet of spacecraft carrying a variety of microwave sensors. The Indian and Chi-nese governments are also rapidly developing spaceborne microwave sensors. Datafrom past and present sensors have been archived and are readily available to re-searchers.

6.1.2.1 Spaceborne Microwave SensorsA variety of active and passive microwave remote sensing instruments have

flown in space. The landmark Seasat-1 mission was probably the most impor-tant early spacecraft-based microwave remote sensing missions (Barrick and Swift,1980; Born et al., 1979). It flew in 1978 and carried a microwave radiometer, analtimeter, a scatterometer, and a SAR. The Seasat-1 mission clearly demonstratedthe utility of these sensors, and lead to multiple follow-on missions.

Some of the most important of these have collected long time series. Theseinclude include the Special Sensor Microwave Radiometer (SMMR) first flownaboard NIMBUS and Seasat-1 in 1978; the Special Sensor Microwave Imager(SSM/I) radiometer (Hollinger et al., 1990, 1987), continuously flown on DefenseMeteorological Satellite Program (DMSP) spacecraft since the early 1980’s; theEarth Resources Satellite (ERS-1/2) Active Microwave Instrument (AMI) (At-tema, 1991), which operated from 1982 through 2001 in scatterometer and SARmodes; the SeaWinds instrument(Spencer et al., 2000) which has operated aboard

6.1. INTRODUCTION 5

QuikSCAT since 1999; the Advanced Microwave Sensor Radiometer (AMSR) op-erating on several spacecraft; the precipitation radar and radiometer aboard theTropical Rain Mapping Mission (TRMM) satellite; the polarimetric WindSat ra-diometer, and a series of ocean altimeters that began with Topex/Poseidon.

A number of synthetic aperture radar (SAR) systems have flown in space in-cluding a SAR aboard Seasat in 1978, the Shuttle Imaging Radar (SIR) series (SIR-A, SIR-B, SIR-C/X-SAR, and SRTM), ERS-1/2, Radarsat-1, and Envisat. Altime-ter missions include the Seasat altimeter, the landmark Topex/Poseidon altimetermission that operated from 1989-2005, the ERS-1/2 altimeters, and Jason-1/2. To-gether these instruments have demonstrated the utility of microwave sensors in thestudy and monitoring of the Earth’s land, ocean, and atmosphere.

For space-based Earth sensing, SAR systems are generally very high resolu-tion (often 100 m and finer) but low coverage while spaceborne radiometers andscatterometers have lower resolution sensors (12 to 75 km) but broader coverage.The resolution of these low resolution sensors is suitable for the oceanic and atmo-spheric applications for which they were designed, but there is growing interest inapplying such microwave sensor data in new applications requiring higher resolu-tion.

Scatterometers operate by transmitting a pulse of microwave energy towardsthe Earth’s surface and measuring the reflected energy. The backscattered energyis related to the normalized radar cross-section (σo) via the radar equation (Ulabyet al., 1981). The spatial response function of the sensor determines the spatialresolution of the σo observation, with typical resolutions varying from 25 to 50 km.Originally designed for retrieval of near-surface winds over the ocean (indirectlyvia measurements of the Bragg scattering from the wind-generated wave field ofthe surface (Naderi et al., 1991; Ulaby et al., 1981; Elachi and van Zyl, 2006)),scatterometer data is now being applied to the study of tropical vegetation, polarice, and global change, e.g. (Ashcraft and Long, 2004; Attema et al., 1998; Forsteret al., 2001; Lecomte et al., 1993; Long and Drinkwater, 1999; Long et al., 2001;Remund and Long, 1999; Wisman et al., 1995). Scatterometer data is also beingoperationally used in weather forecasting and sea-ice monitoring (Ashcraft andLong, 2004).

Radiometers are passive, receive-only sensors which measure the thermal emis-sion (brightness temperature) of the target in the microwave band (Fung, 1994;Tsang et al., 1985; Ulaby et al., 1981; Woodhouse, 2006). The apparent scenebrightness temperature is related to the emissivity and temperature of the surfaceand is modified by moisture content (Kraszewski, 1996) and temperature of theintervening atmosphere. By appropriate selection of operating frequencies in sev-eral microwave bands, the temperature and moisture content of the atmosphere(Jones and Haar, 1990), as well as key surface properties such as land surface tem-


perature (McFarland et al., 1990), soil and plant moisture (Jackson and Schmugge,1989; Pampaloni and Paloscia, 1986), sea-ice mapping (Thomas et al., 1985), snowcover classification (Grody et al., 1991), and wind speed (over the ocean) (Wentz,1991), can be retrieved. Radiometer data is being operationally used in weatherforecasting and sea-ice monitoring. Radiometers are also used for correcting pathlength variations in altimetry (Bernard et al., 1993; Fu et al., 1994; Ruf et al.,1995). Ground-based radiometers are used for measuring meteorological variables(Janssen, 1993) and radio astronomy (Burke and Graham-Smith, 2002; Verschuuret al., 1987; Rohlfs and Wilson, 1996). Microwave limb sounders are specializedradiometers which measure vertical profiles of atmospheric brightness temperatureby looking horizontally through the atmosphere. From this data, vertical profiles oftemperature, pressure and moisture can be extracted. Limb sounders are not treatedin this chapter.

Altimeters are ranging radars designed to measure the distance from a space-craft or aircraft to the ground. With a carefully measured spacecraft orbit, heightaccuracies to a few cm are possible (Fu et al., 1994). From the altimeter heightmeasurements, surface topography can be measured. Over the ocean the surfacetopography can be related to ocean currents and undersea topography. Other in-formation such as ocean wave height and near-surface wind speed can also beextracted. Altimeter data has also been used for creating digital elevation maps ofpolar regions, e.g., (Bamber, 1994; Zwally et al., 1983).

Weather radars are ranging scatterometers designed to measure rainfall (Doviaket al., 1979; Doviak and Zrnic, 1984; Meneghini and Kozu, 1990). Some weatherradars can measure radial velocity and thus can determine wind as well as rain.Ground-based weather radars (i.e., NEXRAD) have been extensively deployed anddata from them are often displayed on television news and weather broadcasts.To-date, the only space-based orbital weather radars have been the Tropical RainMapping Mission (TRMM) Precipitation Radar (PR) and Cloudsat. TRMM alsocarried a radiometer, the TRMM Microwave Imager (TMI) (Awaka et al., 1997;Meneghini et al., 2000).

6.2 Passive Systems (Radiometers)

6.2.1 Background

Microwave radiometers measure the thermal emission, sometimes called the Plankradiation, radiating from natural objects (Ulaby et al., 1981; Woodhouse, 2006).As passive sensors they require no transmitter but are receive-only. As a result,radiometers require less power to operate than radars.

6.2. PASSIVE SYSTEMS (RADIOMETERS) 7

6.2.1.1 Radiometer FundamentalsIn a typical radiometer, an antenna is scanned over the scene of interest and the

output power from the carefully calibrated receiver is measured as a function ofscan position. The observed power is related to the receiver gain and noise figure,the antenna loss, the physical temperature of the antenna, the antenna pattern, andthe scene brightness temperature. In simplified form, the output power PSYS ofthe receiver can be written,

PSYS = kTSYSB (6.1)

where k = 1.38× 10−28 is Boltzmann’s constant, B is the receiver bandwidth andTSYS is the system temperature,

TSYS = ηlTA + (1 − ηl)Tp + (L − 1)Tp + LTREC (6.2)

where ηl is the antenna loss efficiency, Tp is the physical temperature of the antennaand waveguide feed, L is waveguide loss, TREC is the effective receiver noisetemperature (determined by system calibration), and TA is the effective antennatemperature. The effective antenna temperature is dependent on the direction theantenna points and the scene characteristics.

The effective antenna temperature, TA can be modeled as a product of theapparent temperature distribution TAP (θ, φ) in the look direction θ, φ (see Fig. 6.3)the antenna radiation gain F (θ, φ) which is proportional to the antenna gain patternG(θ, φ).

TA (in K) is obtained by integrating the product of apparent temperature dis-tribution TAP (θ, φ) (in K) at the antenna pattern G(θ, φ):

TA =1

G

∫ ∫

G(θ, φ)TAP (θ, φ)dθdφ (6.3)

whereG =

∫ ∫

G(θ, φ)dθdφ (6.4)

where integrals are over the range of values corresponding to the non-negligiblegain of the antenna. Note that the antenna pattern acts as a low pass filter of thesurface brightness, limiting the effective spatial resolution of the measurement toapproximately the 3 dB beamwidth. The observed value can be split into contribu-tions from the mainlobe and the sidelobes,

Ta = ηMTML + (1 − ηM )T SL (6.5)

where ηM is the main lobe efficiency factor

ηM =1

G

∫ ∫

main lobeGi(θ, φ)dθdφ (6.6)


TbTsc

Tsky

sec�

Tup

AntennaPattern G(

�� )

AntennaTA

AntennaTemperatureDistributionTAP (

�� )

Surface

Atmosphere

Figure 6.3: The apparent temperature distribution and some of its contributions.The antenna temperature TA is the normalized integral of product of the tempera-ture distribution and the antenna gain pattern. The apparent temperature of the sur-face seen through the atmosphere includes the upwelling radiation, Tup, from theatmosphere plus the attenuated surface brightness temperature, Tb, and the surface-scattered brightness temperature, Tsc. Brightness temperature contributions fromthe extra-terrestrial sources are grouped in Tsky.

and

TML =1

G

∫ ∫

main lobeG(θ, φ)TAP (θ, φ)dθdφ (6.7)

T SL =1

G

∫ ∫

side lobesG(θ, φ)TAP (θ, φ)dθdφ. (6.8)

For downward -looking radiometers, the apparent brightness temperature dis-tribution includes contributions from the surface and the intervening atmosphere(Ulaby et al., 1981). For a spaceborne sensor this can be expressed as,

TAP (θ, φ) = [Tb(θ, φ) + Tsc(θ, φ)]e−τ sec θ + Tup(θ) (6.9)

6.2. PASSIVE SYSTEMS (RADIOMETERS) 9

where Tb(θ, φ) is the surface brightness temperature, Tsc(θ, φ) is the surface scat-tering temperature, τ is the total effective optical depth of the atmosphere and Tup

is the effective atmospheric upwelling temperature. Tup is the effective radiomet-ric temperature of the atmosphere which depends on the temperature and densityprofile, atmospheric losses, clouds, etc. (Janssen, 1993). The situation is modifiedsomewhat for upward-looking radiometers since there is little surface contribution.

Ignoring incidence and azimuth angle dependence, the surface brightness tem-perature is given by,

Tb = εTP (6.10)

where ε is the emissivity of the surface and TP is the physical temperature of thesurface. The emissivity is a function of the surface roughness and the permittivityof the surface which are related to the geophysical properties of the surface (Ulabyet al., 1981).

The surface brightness temperature, Tsc(θ, φ), is the result of downwelling at-mospheric emissions which are scattered off of the rough surface toward the sensor.This signal depends on the scattering properties of the surface (surface roughnessand dielectric constant) as well as the atmospheric emissions directed toward theground. Note that azimuth variation with brightness temperature has been observedover the ocean (Wentz, 1992), sand dunes (Stephen and Long, 2005), and snow inAntarctica (Long and Drinkwater, 2000).

The total effective optical depth of the atmosphere, τ , corresponds to the sig-nal loss from the surface vertically through the atmosphere. This depends on thefrequency, density profile, and other atmospheric factors. Given the atmosphericdensity, temperature, cloud and liquid water profile, τ , Tsc, and Tup can be com-puted using radiative transfer techniques (Ulaby et al., 1981). Note that these aredependent on frequency and polarization. By making multiple channel (i.e. sev-eral measurements at different frequencies and electromagnetic polarizations), keyparameters of the atmospheric profile can be determined by inverting the radiativetransfer equations (Cherny and Raizer, 1998; Janssen, 1993; Ulaby et al., 1981).

Radiometer measurements are “noisy” due to the limited integration time avail-able for each measurement. The uncertainty is expressed as ∆T , which is the stan-dard deviation of the temperature measurement. ∆T is a function of the integrationtime and bandwidth used to make the radiometric measurement and is typically in-versely related to the time-bandwidth product (Ulaby et al., 1981). Increasing theintegration time and/or bandwidth reduces ∆T . High stability and precise calibra-tion of the system gain is required to accurately infer the brightness temperature Tb

from the sensor power measurement PSYS.A wide variety of techniques are used to provide the required calibration (Ulaby

et al., 1981). A Dicke radiometer periodically switches a known power noise


source into the receiver to measure the gain. Total power radiometers avoid theswitching but use antenna rotation to point the antenna at deep space (which is cold)and/or warm calibration targets to provide reference calibration measurements.

6.2.1.2 Polarimetric RadiometryIn polarimetric radiometry multiple polarization emissions are measured and

cross-correlated. In polarimetric radiometry the observed brightness temperatureTb,p at polarization p is related to the physical temperature (radiance) TP by thepolarization-dependent emissivity εp according to

Tb,p = εp(θ, φ)TP (6.11)

where θ and φ are the viewing geometry parameters. The emissivity depends onthe physical properties of the medium, including the dielectric constant and sur-face roughness. In this expression, p can be v, h, 45, −45, lc, or rc, correspondingto vertical, horizontal, plus 45◦, minus 45◦, left-hand circular, and right-hand cir-cular polarizations, respectively. These polarizations are computed by taking theappropriate averages of the observed electric field correlations.

The modified Stokes vector Is,

Is =

I

Q

U

V

=

Tv

Th

T45 − T−45

Tlc − Trc

=

〈EvE∗

v 〉〈EhE∗

h〉2Re〈EvE

∗

h〉2Im〈EvE

∗

y〉

(6.12)

where the 〈〉 denotes averaging of the enclosed emitted electric fields, providesa full characterization of the electromagnetic signature of the surface. The vari-ous terms (I , Q, U , V ) depend on both the electrical and mechanical properties(roughness) of the scene (Piepmeier et al., 2007).

An important application of polarimetric radiometry is in ocean wind retrieval(Yueh et al., 1994). The microwave emission from the ocean is wind speed de-pendent due to the roughening of the surface caused by wind-generated waves.The wave structure also leads to (small) sinusoidal dependence of the emission onthe wind direction. While the H and V components of the Stokes vector are inphase, the third (U ) and (V ) components are out-of-phase with the H and V com-ponents, making it possible to retrieve the wind direction with a single azimuthlook. Because of the small modulation (a few degrees K), however, the U and V

components must be very precisely measured (Gaiser et al., 2004; Hudson et al.,2007; Piepmeier et al., 2007).

6.3. EARTH OBSERVING SPACEBORNE RADIOMETER SYSTEMS 11

6.2.1.3 Scanning MethodsThe resolution of a radiometer is defined by the antenna gain pattern, specifi-

cally the extent of the mainlobe. Motion during the integration period can smearand degrade the resolution. Radiometric interferometers employ multiple antennasand use the interference patterns to improve the resolution (Wohlleben et al., 1991).In either case, to provide greater angular and/or spatial coverage, the antenna isscanned, either mechanically or electrically. A number of scanning schemes areemployed on spacecraft for Earth remote sensing. The three most important aremultiple fixed beams, cross-track scanning, and helical scanning, illustrated inFig. 6.4. In a helical scanning sensor, the antenna is pointed off-nadir and rotatedabout the nadir vector. As the spacecraft moves, the ground trace of the antennaboresight forms a helix. This is the most common scanning geometry for space-borne radiometers. It has also been used for the SeaWinds scatterometer. Syntheticaperture radiometers are being developed (Ruf et al., 1988).

6.2.2 Airborne SystemsA large number of radiometer systems have been operated from various aircraft,mostly to support various experiments or as demonstration prototypes for space-borne sensors. Some, however, are in operational use. For example, NOAA oper-ates the AMSU system aboard its hurricane hunter aircraft to sample and observeweather conditions on its flight into hurricanes. While airborne systems are of greatinterest, due to limitations of space, they are not considered in this chapter.

6.2.3 Spaceborne SystemsSpaceborne radiometers systems have provided global observations of the earthsince the late 1970’s and continue to be an important tool for the study of the earth.All though the space environment is severe, with proper design practices systemswith the high calibration precision and stability required to ensure the utility ofthe data. Starting with early scanning systems radiometer technology continues toevolve with the recent addition of fully polarimetric systems such as WindSat. Anumber of important historical and contemporary spaceborne radiometer systemsare described in the next section.

6.3 Earth Observing Spaceborne Radiometer SystemsThe goal of this subsection is to summarize basic sensor information about anumber of important Earth-looking radiometer systems that have operated aboardspacecraft. Only selected sensors are included.


Radiometer Electronics

Oscillating

Reflector

Flight Direction

RadiometerElectronics

Flight

Direction

Rotating

Reflector

Radiometer

ElectronicsFlight

Direction

(a)

(b)

(c)

Figure 6.4: Illustration of the main radiometer scanning types: (a) Multiple fixedbeams in a “push-broom” configuration; (b) Cross-track scanning with the aid ofa scanning reflector; and (c) Helical scanning, which results from spinning theantenna about the nadir axis.


6.3.1 Historical RadiometersA significant number of short-term experimental radiometer systems have beenflown; however, this data is not generally available and so they are not included inthis listing. Instead, only the most important of these is listed here.


6.3.1.1 Scanning Multichannel Microwave Radiometer (SMMR)The SMMR instrument is a five frequency radiometer that first flew on Nimbus-

7 and later flew aboard Seasat, both launched in 1978. The inherent resolution ofthe various SMMR channels varies from a coarse 95 km × 148 km to as fine as18 km × 27 km depending on frequency (Gloersen and Barath, 1977; Njoku et al.,1980). The SMMR was the first successful spaceborne helical scanning radiometerand data from SMMR data has been widely used in a variety of studies, particularlyof sea ice properties, including extent and thickness (Gloersen et al., 1987).

Table 6.1: Sensor characteristics of SSMR.Sensor SMMRSensor Type Dicke RadiometerSensor Scan helical scanPolarization H, VCenter Frequency 6.63, 10.69, 18.0, 21.0, 37.0 GHzOperating Bandwidth 250 MHzRadiometric Accuracy (∆T ) 0.4 – 1.1Coverage globalSwath Width 780 kmFootprint Shape ellipticalFootprint Size 121×79 km, 74×49 km,(Seasat) 44×29 km, 38×24 km,

21×14 kmIncidence Angle Nimbus-7: 53.3◦ , Seasat: 49◦

Mission Length Nimbus-7: 1978-1987, Seasat: 1978Spacecraft Nimbus-7, SeasatOrbit ∼ 800 km circular sun-synchronousFundamental Measurement Brightness temperature (Tb)Key Products Sea ice extent and concentration

water vaporatmospheric liquid water

Product Resolution 25 kmData Availability podaac.jpl.nasa.gov

nsidc.orgeosweb.larc.nasa.gov


6.3.2 Contemporary RadiometersA number of important radiometer sensors or series of sensors are currently operat-ing. Data from these sensors are generally available for scientific research. Whileit is not possible to list all sensors, a number of significant sensors are described.

6.3.2.1 Special Sensor Microwave/Imager (SSM/I)The SSM/I is a total-power radiometer with seven channels. These channels

cover four different frequencies with horizontal and vertical polarizations channelsat 19.35, 37.0, and 85.5 GHz and a vertical polarization channel at 22.235 GHz(Hollinger et al., 1990). An integrate-and-dump filter is used to make radiomet-ric brightness temperature measurements as the antenna scans the ground trackvia antenna rotation (Holliner, 1989). The 3 dB elliptical antenna footprints range

Swath1394 km

SubsatelliteTrack

102°

Figure 6.5: An illustration of the SSM/I coverage swath, which consists of a portionof the helical scan of the antenna. The remainder of the scan is used for calibration.The dark ellipse schematically illustrates the antenna mainlobe on the surface fora particular channel which is maintained at a constant incidence angle.

from about 15-70 km in the cross-scan direction and 13-43 km in the along-scandirection depending on frequency (Hollinger et al., 1987). First launched in 1972,SSM/I instruments have flown on multiple spacecraft continuously until the present(2008) on the Defense Meteorological Satellite Program (DMSP) (F) satellite se-


ries. The scanning geometry results in a swath diagram as shown in Fig. 6.5. Withits multiple sensors, the extensive SSM/I dataset provides a critical database forstudies of long-term climate change, including atmospheric moisture, rainfall, air-sea interaction, and surface wind speed (see, for example, (Berg and Kummerow,2005; Colton and Poe, 1999; Wentz, 1991)). Though originally designed for atmo-spheric sensing, SSM/I data has been extensively used for studies of sea ice andother applications (Ferraro et al., 1996).

Table 6.2: Sensor characteristics of SSM/I.Sensor SSM/ISensor Type RadiometerSensor Scan helical scanPolarization 19.35, 37, 85.5 : H, V

22.23 : VCenter Frequency 19.35, 22.23, 37, 85.5 GHzOperating Bandwidth 125, 300, 750, 500, 2000 MHzRadiometric Accuracy (∆T ) 1.0 to 0.3Coverage globalSwath Width 1400 kmFootprint Shape ellipticalFootprint Size 43×69 km, 43×69 km,

60×40 km, 37×28 km,37×20 km, 15×13 km,15×13 km

Incidence Angle ∼ 53◦, 49.9◦ for 10.7 GHzMission Length 1972 to presentSpacecraft DMSP F seriesOrbit 805 km circular sun-synchronousFundamental Measurement Brightness temperature (Tb)Key Products wind speed

atmospheric temperaturerain ratewater vapor

Product Resolution 25 km, 12.5 kmData Availability www.remss.com

www.ncdc.noaa.govrain.atmos.colorstate.edu/CRDC


6.3.2.2 Microwave Sounding Unit (MSU)A number of MSU radiometer instruments have been operated on NOAA polar-

orbiting platforms. Using fixed beam scanning, the MSU is designed for for atmo-spheric temperature profiling at multiple incidence angles. An extensive databasestarting from 1979 through the present has been collected.

Table 6.3: Sensor characteristics of MSU.Sensor MSUSensor Type microwave sounderSensor Scan multiple fixed beams

11 cross-track observation pointsCenter Frequency 50.3–57.95 GHzCoverage globalIncidence Angle 0, 11, 22, 33, 44 and 55 degMission Length 1979 to presentSpacecraft TIROS-N NOAA Polar platformsFundamental Measurement Brightness temperature (Tb)Key Products atmospheric temperature profilesData Availability www.remss.com

www.doc.jaxa.jp


6.3.2.3 Advanced Microwave Sounding Unit (AMSU)AMSU is a multi-channel microwave sounder designed primarily to obtain

temperature and humidity profiles in the upper atmosphere (especially the strato-sphere) and to provide a cloud-filtering capability for tropospheric temperatureobservations. The first AMSU was launched in May 1998 on board the DMSPF-15 (now NOAA 15) satellite and has flown on NOAA 16 and 17l. AMSU issimilar to MSU but includes additional channels. An advanced version known asthe Advanced Technology Microwave Sounder (ATMS) is planned for the NPOESmission (Goldberg et al., 2006).

AMSU consists of two instruments: AMSU-A with 15 channels and AMSU-B with 5 channels. AMSU-A is designed primarily for atmospheric temperatureprofiling and thus has many narrow channels. AMSU-B is designed for moistureprofiling, but has other applications. For example, the observations can also befor land surface cover studies (Karbou, 2005; Karbou et al., 2005) and snow cover(Kongoli et al., 2003).

Table 6.4: AMSU-A & AMSU-B Frequencies (GHz) and ChannelsChannel AMSU-A AMSU-B1 23.8 89.02 31.4 150.03 50.3 183.3±14 52.8 183.3±35 53.6 183.3±76 54.47 54.98 55.59 57.210-14 57.29 ±.217 & ±.32215 89.0

Notation: x±y where x is the center frequency. If y appears, the measurementsare made in two bands on either side of the center frequency and y is the distance

of the passbands to the center frequency[http://amsu.cira.colostate.edu].


Table 6.5: Sensor characteristics for AMSU.Sensor AMSUSensor Type microwave sounderSensor Scan cross-track scanCenter Frequency AMSU-A: 15 channels 15-90 GHzCoverage globalSwath Width 1650 kmFootprint Shape variable, ellipticalMission Length 1998 to presentSpacecraft NOAA platforms

NASA Aqua platformOrbit 833 km circular sun-synchronous

101 min period98.7◦ inclination angle

Fundamental Measurement Brightness temperature (Tb)Key Products atmospheric temperature profilesProduct Range 5 levels to 40 kmData Availability www.remss.com

www.orbit.nesdis.noaa.govamsu.cira.colostate.edughrc.msfc.nasa.gov


6.3.2.4 Tropical Rain Measuring Mission (TRMM) Microwave Imager (TMI)TMI was launched in 1995 as part of the TRMM mission (Awaka et al., 1997;

Meneghini et al., 2000). The conically scanning TMI measured radiometric emis-sions in several microwave channels and polarizations designed for measuring at-mospheric moisture and rain over a wide swath. TRMM has far exceeded its de-sign life and has provided an extensive data set over the tropics (latitude less than± 30◦). Designed for a 350 km altitude, the TRMM orbit altitude was raised in2001 to extend the mission life. TMI data is augmented by TRMM PR data. TheTMI 3 dB elliptical antenna footprints range from about 5-8 km in the cross-scandirection and 7-63 km in the along-scan direction depending on frequency. Thescanning geometry results in the swath diagram as shown in Fig. 6.6 at the 350 kmorbit height.

Swath759 km

SubsatelliteTrack

130°

Figure 6.6: An illustration of the TMI coverage swath, which consists of a portionof the helical scan of the antenna. The remainder of the scan is used for calibration.The observation incidence angle is essentially constant.


Table 6.6: Sensor characteristics for TMI.Sensor TMISensor Type RadiometerSensor Scan helical scanPolarization V & H (except 21.3 GHz which V only)Center Frequency 10.7, 19.4, 21.3, 37, 85.5 GHzCoverage 40S to 40NSwath Width 780 kmFootprint Shape ellipticalFootprint Size 45 km to 5 kmIncidence Angle 52.8◦

Mission Length Dec. 1995 to presentSpacecraft TRMMOrbit 350 & 400 km circular, semi-equatorial

35◦ inclination anglenon-sun synchronous

Fundamental Measurement Brightness temperature (Tb)Key Products sea surface temperature

ocean wind speedwater vaporcloud waterrain ratewater vapor

Product Resolution 25 kmData Availability gsfc.nasa.com

www.remss.com


6.3.2.5 Microwave Scanning Radiometer (MSR)The MSR flew aboard the Japanese Marine Observation Satellite-1 (MOS-1)

launched in 1987 and aboard the follow-on MOS-1b in 1990 (Maeda et al., 1989).MSR data can be used infer to sea surface temperature (SST) (Takeuchi et al.,1993).

Table 6.7: Sensor characteristics for MST.Sensor MSRSensor Type RadiometerSensor Scan helical scanPolarization H, VCenter Frequency 12, 31 GHzCoverage globalSwath Width 320 kmFootprint Shape ellipticalMission Length MOS-1: Feb. 1987 – Nov. 1995

MOS-1: Feb. 1990 – Apr. 1996Spacecraft MOS-1, MOS-1bOrbit 909 km circular sun-synchronous

103 min period99◦ inclination angle

Fundamental Measurement Brightness temperature (Tb)Key Products sea surface wind speed

sea surface temperatureintegrated water vaporsnow fall

Product Resolution 32 km, 23 kmData Availability www.eorc.jaxa.jp

www.doc.jaxa.jp


6.3.2.6 Advanced Microwave Scanning Radiometer (AMSR and AMSR-E)The Japanese Aerospace Exploration Agency (JAXA) developed the design for

AMSR for the ADEOS-II mission. AMSR first flew on ADEOS-II which operatedfrom Jan. 2003 through Oct. 2003 before the mission prematurely terminateddue to loss of spacecraft power. A second instrument, denoted AMSR-E, waslaunched aboard the U.S. Aqua mission. AMSR-E is similar to AMSR but doesnot include 50 MHz channels (Kwanishi et al., 2003). AMSR-E employs a 1.6 moffset parabolic dish antenna which is conically scanned. Some radio frequencyinterference has been observed in the 6.9 GHz and 10.65 GHz channels (Li et al.,2004; Njoku et al., 2005). The data is widely used for a variety of oceanographic,atmospheric, cryosphere, and land studies. The AMSR scanning geometry resultsfixed incidence angle observations over the swath as diagramed in Fig. 6.7

Swath1445 km

SubsatelliteTrack

122°

Figure 6.7: An illustration of the AMSR coverage swath, which consists of a por-tion of the helical scan of the antenna. The remainder of the scan is used forcalibration. The observation incidence angle is essentially constant.


Table 6.8: Sensor characteristics for AMSR and AMSR-E.Sensor AMSR & AMSR-ESensor Type RadiometerSensor Scan helical scanPolarization H, V

AMSR: 50.3 and 52.8 are V-onlyCenter Frequency 6.925, 10.65, 18.7, 23.8, 36.5, 89.0 GHz

AMSR: 50.3 and 52.8 GHzOperating Bandwidth 360,100,200,400, 1000, 3000 MHzRadiometric Accuracy (∆T ) 0.3 K, 0.6 K, 0.6 K, 0.6 K, 0.6 K, 1.1K

AMSR: 2K for 50.3 and 52.8 GHzCoverage globalSwath Width AMSR 1600 km

AMSR-E 1450 kmFootprint Shape ellipticalFootprint Size 75×43 km, 51×29 km,

27×16 km, 32×18 km,14×8 km, 6×4 km

Incidence Angle ∼ 55◦

Mission Length AMSR: Jan-Oct 2003AMSR-E: May 2002 to present

Spacecraft AMSR: ADEOS-II (Midori-2)AMSR-E: Aqua

Orbit AMSR: 805 km circular sun-synchronousAMSR-E: 705 km circular sun-synchronous

Fundamental Measurement Brightness temperature (Tb)Key Products sea surface wind speed

sea surface temperatureintegrated cloud liquid waterprecipitationsea ice concentrationintegrated water vaporsnow depthsoil moisture

Data Availability www.eorc.jaxa.jpwww.remss.com


6.3.2.7 WindSatThe WindSat/Coriolis mission carries the first orbital polarimetric radiome-

ter. The WindSat radiometer is the testbed for a new series of radiometers to beflown aboard the National Polar-orbiting Operational Environmental Satellite Sys-tem (NPOESS) (Gaiser et al., 2004). WindSat is designed to evaluate the viabilityof polarimetric radiometry for measuring the speed and direction of ocean windsfrom space. WindSat includes multiple polarimetric and dual-polarized channelssharing a 1.8 m offset reflector antenna. The scanning geometry was selected toevaluate both single look and dual azimuth retrieval of wind speed and direction(Gaiser et al., 2004). The data can also be used to observe sea surface tempera-ture, rain, cloud liquid water, soil moisture, and other variables (Lee et al., 2005;Windsat, 2007).

The scanning geometry of WindSat is designed to make both forward and aft-facing Tb measurements over part of its observation swath (see Fig. 6.8) in orderto evaluate the potential of using the azimuth dependence of Tb to retrieve near-surface ocean winds from the measurements.


Forward Swath1025 km

SubsatelliteTrack

40° 28°

10°

23°

Aft Swath390 km

Figure 6.8: An illustration of the WindSat overvation geometry and coverage asseen from above. The antenna helically scans the surface at a constant incidenceangle. The remainder of the azimuth angles are used for calibration.


Table 6.9: Sensor characteristics for WindSat.Sensor WindSatSensor Type Polarimetric radiometerSensor Scan helical scanPolarization 10.7, 18.7, 37 : H, V, ±14, L, R

6.8, 23.8 : H, VCenter Frequency 6.8, 10.7, 18.7, 23.8 37.0 GHzOperating Bandwidth 125, 300, 750, 500, 2000 MHzRadiometric Accuracy (∆T ) 0.05 K (12 sec integration)Radiometric Stability (±T ) 0.12 K (8 days)Coverage globalSwath Width 1025 kmFootprint Shape ellipticalFootprint Size 40×60 km, 25×38 km,

16×27 km, 12×20 km,8×13 km

Incidence Angle ∼ 53◦, 49.9◦ for 10.7 GHzMission Length Jan. 2003 to presentSpacecraft CoriolisOrbit 830 km circular sun-synchronousFundamental Measurement Brightness temperature (Tb)Key Products ocean wind vectors (primary)

sea surface temperaturesoil moisturerain ratewater vapor

Product Resolution 25 kmData Availability podaac.jpl.nasa.gov

manati.orbit.nesdis.noaa.govwww.nrl.navy.mil/WindSat


6.3.2.8 Multi-frequency Scanning Microwave Radiometer (MSMR)The MSMSR a four frequency dual-polarization radiometer that is broadly

similar to SMMR. Developed by India, it was first launched on the India Re-search Satellite (IRS) P-4 in May 1999. Another MSMR flys onboard the IndianOCEANSAT-1 (Bhandari et al., 2002; Misra et al., 2002). MSMR has been usedextensively for the study of sea ice (Vyas and Dash, 2000; Vyas et al., 2004).

Table 6.10: Sensor characteristics of MSMR.Sensor MSMRSensor Type RadiometerSensor Scan helical scan, 11.173 RPMPolarization H, VCenter Frequency 6.6, 10.65, 18, 21 GHzCoverage globalSwath Width 1350 kmFootprint Shape ellipticalIncidence Angle 49.7◦

Mission Length 1999 to presentSpacecraft IRS P-4, OCEANSAT-1Orbit 720 km circular sun-synchronous

98.28◦ inclination angle12:00 ascending node

Fundamental Measurement Brightness temperature (Tb)Key Products wind speed

atmospheric temperaturerain ratewater vapor

Product Resolution 150, 75, 50, 60 kmData Availability www.remss.com


6.3.3 Planned Radiometer SystemsMany new spaceborne passive microwave sensors are in various planning stages inthe U.S., Japan, Europe, China, and India. Some of these are extensions of existingmissions, but many new missions are also planned. While it is not possible toinclude all of these, several important planned systems are described.

6.3.3.1 AquariusThe Aquarius mission is a joint mission between NASA, which is supplying

the sensors, and Argentina, which is suppling the spacecraft. The Aquarius mis-sion includes both an L-band polarimetric radiometer operating at 1.4 GHz and adual-frequency L-band scatterometer operating at 1.2 GHz to make precise globalobservations of sea surface salinity from space (Aquarius, 2007). Both sensorsshare a 2.5 m reflector that employs a helical scanning pattern. Aquarius is sched-uled for launch in Sept. 2008. Key instrument parameters are given in the followingtables.


Table 6.11: Sensor characteristics for the Aquarius Radiometer.Sensor Aquarius RadiometerSensor Type Polarimetric radiometerSensor Scan 3 beam helical scanPolarization H, V, UCenter Frequency 1.413 GHzOperating Bandwidth 4 MHzNet integration time 20 ms and 10 sRadiometric Accuracy (∆T ) 0.05 K (12 sec integration)Radiometric Stability (±T ) 0.12 K (8 days)Coverage global, every 8 daysSwath Width 340 kmFootprint Shape ellipticalFootprint Size 62×68 km, 68×82 km, 75×100 kmIncidence Angle 23.3◦, 33.7◦ , 41.7◦

Mission Length Sept. 2008 to Sept. 2011Orbit 600 km circular sun-synchronous

98◦ inclination angle6 pm descending node

Fundamental Measurement Brightness temperature (Tb)Key Products* Monthly ocean salinity mapProduct Accuracy 0.2 psuProduct Range 32-37 pptData Availability (future)

* Aquarius mission product (combined radiometer/scatterometer)


Table 6.12: Sensor characteristics for the Aquarius Scatterometer.Sensor Aquarius ScattterometerSensor Type Polarimetric scatterometerSensor Scan single beam helical scanPolarization H, VCenter Frequency 1.413 GHzPulse Length 1 msPRF 100 HzOperating Bandwidth 4 MHzRadiometric Accuracy (Kp) 0.1 dBRadiometric Stability (±σo) 0.13 dB (8 days)Coverage global, every 8 daysSwath Width 340 kmFootprint Shape ellipticalMission Length Sept. 2008 to Sept. 2011Orbit 600 km circular sun-synchronous

98◦ inclination angle6 pm descending node

Fundamental Measurement radar backscatter (σo)Key Products* Monthly salinity mapProduct Accuracy 0.2 psuData Availability (future)* Aquarius mission product (combined radiometer/scatterometer)


6.3.3.2 Soil Moisture and Ocean Salinity (SMOS)The SMOS mission will use an L-band 2-d interferometric radiometer to make

high resolution measurements of soil moisture and ocean salinity (Font et al., 2000;Kerr et al., 2001). This innovative sensor is scheduled for launch in 2008.

Table 6.13: Sensor characteristics for SMOS.Sensor SMOSSensor Type Interferometric radiometerPolarization H, V, UCenter Frequency 1.41 GHzFundamental Measurement Brightness temperature (Tb)Radiometric Accuracy (∆T ) 0.64 KCoverage globalSwath Width 1500 kmFootprint Shape ellipticalFootprint Size nominally 40 kmMission Length 2008-2011Spacecraft ProteusOrbit 763 km circularKey Products soil moisture

ocean salinityProduct Resolution 30-50 kmProduct Accuracy 0.1 psu (salinity)Data Availability (future)


6.3.3.3 HydrosThe NASA Hydrosphere State (Hydros) mission was to include both an inno-

vative scanning synthetic aperture radar/scatterometer and a radiometer in orderto measure soil moisture from orbit (Njoku et al., 2003). Hydros was scheduledfor launch in 2010, but has been canceled. It is likely to be rescheduled under adifferent name.

The mission was to include both an L-band radiometer operating at 1.4 GHzand an L-band SAR/scatterometer operating at 1.2 GHz to make precise global ob-servations of sea surface salinity from space (Aquarius, 2007). Both sensors sharea horn feed and 6 m reflector that is helically scanned. Key instrument parametersare given in the following tables.

Soil moisture is retrieved from the radiometer measurements with correctionsfor soil roughness obtained from the scatterometer (Njoku and Entekhabi, 1996;Njoku et al., 2000). The scatterometer design is based on SAR-type processingof scanning scatterometer data, though low-resolution processing is also planned(Spencer et al., 2003).

Table 6.14: Sensor characteristics for the proposed Hydros Radiometer.Sensor Hydros RadiometerSensor Type Polarimetric radiometerSensor Scan single beam helical scanPolarization H, V, UCenter Frequency 1.41 GHzFundamental Measurement Brightness temperature (Tb)Radiometric Accuracy (∆T ) 0.64 KCoverage globalSwath Width 1000 kmFootprint Shape ellipticalFootprint Size nominally 40 kmMission Length Sept. 2011 to 2013Orbit 670 km circular sun-synchronous 6 amKey Products* Monthly soil moisture map

Freeze/Thaw stateProduct Resolution 3×3 km and 10×10 kmProduct Accuracy ±4%Data Availability (future)

* mission product (combined radiometer/scatterometer)


Table 6.15: Sensor characteristics for the proposed Hydros Scatterometer.Sensor Hydros ScattterometerSensor Type Polarimetric scatterometerSensor Scan single beam helical scanPolarization HH, VV, HVCenter Frequency 1.26 GHzFundamental Measurement radar backscatter (σo)Fundamental Resolution 3 km, 10 km, 30 kmRadiometric Stability (±σo) 1.0 dB (3 km), 0.45 dB (10 km)Coverage globalSwath Width two 350 km wide swaths

separated by 300 km wide gapFootprint Shape ellipticalFootprint Size 30 kmIncidence Angle 39.3◦

Mission Length 2011 to 2013Orbit 670 km circular sun-synchronous 6 amKey Products* Monthly soil moisture map

Freeze/Thaw stateProduct Resolution 3×3 km and 10×10 kmProduct Accuracy ±4%Data Availability (future)

* mission product (combined radiometer/scatterometer)

6.3.3.4 Conical Microwave Imager Sounder (CMIS)CMIS is a multichannel helically scanning radiometer originally planned for

deployment as part of the National Polar-orbiting Operational Environmental Satel-lite System (NPOESS). Due to program cost overruns, CMIS has been descoped.While it is very likely that NPOESS will include a radiometer, it it not clear pre-cisely what features it will have.

6.3.3.5 Advanced Technology Microwave Sounder (ATMS)ATMS is the next generation of AMSU instruments and includes some addi-

tional channels for a total of 22. A flat, rotating reflector is planned (Goldberg etal., 2006).

6.3.3.6 Global Precipitation Measurement Program (GPM)The Japanese GPM mission will include a Dual-Frequency Precipitation Radar

(DPR) and a Microwave Radiometer Imager (GMI). The DPR consists of a TRMM/PRweather radar operating at 13.6 GHz with an added 34.5 GHz precipitation radar.


The 13.6 GHz channel will have a swath width of 245 km while the 34.5 GHzchannel will have a narrower swath of about 100 km.

GMI will be a nine channel radiometer similar to the TRMM Microwave Im-ager (TMI), but with fewer channels and a smaller (1.2 m) antenna. It will havea 800 km wide swath. GMI is designed to support rain observation and will bedeployed on the GPM satellite operating at 400 km altitude.

6.3.4 Extra-Terrestrial Radiometer SensorsWhile most radio astronomy has been done from ground-based radio telescopes,several spaceborne radiometers systems have flown to observe extra-terrestrial phe-nomena. Of particular note are missions to map the cosmic background radiationat microwave frequencies. The cosmic background radiation is a low level (about2.7 K) brightness temperature that permeates deep space. It is believed to originatefrom the big bang (Bennett et al., 1996). Originally thought to be isotropic, veryprecise radiometers have found very small anisotropies in the cosmic backgroundradiation wich are thought be the result of turbulence in the early universe (Levi,1992). Several microwave sensors designed to map these anistropies in the cosmicbackground radiation are described below.


6.3.4.1 COBE DMRThe Cosmic Background Explorer (COBE) mission carried a number of sen-

sors to study the cosmic background radiation (CBR). One of its key sensors wasthe Differential Microwave Radiometers (DMR), which consists of six differen-tial radiometers that operate in pairs (Smoot et al., 1990). Pointing in differentdirections, the difference between the signals in each antenna is recorded to mapanisotropy in the CBR. To achieve the extreme precision, a careful differentialradometric measurement scheme is used for the recievers.

Table 6.16: Sensor characteristics for COBE.Sensor COBE DMRSensor Type paired differential radiometerSensor Scan precessing helical scanPolarization circularCenter Frequency 31.5, 53, & 90 GHzFundamental Measurement Differential brightness temperature (dTb)Coverage full-skyMission Length 1989 to 1993Spacecraft COBEOrbit 900 km circular polarKey Products* Differential Tb mapProduct Accuracy 3.3 mK%Data Availability lambda.gsfc.nasa.gov/product/cobe

* one of several mission products

6.3.4.2 ReliktThe Soviet Prognoz 9 mission included the Relikt-1 radiometer to measure the

CMB using a single channel radiometer (Strukov et al., 1992).

Table 6.17: Sensor characteristics for Relikt.Sensor ReliktSensor Type radiometerCenter Frequency 37 GHzFundamental Measurement brightness temperature (Tb)Coverage full-skyMission Length 1983-1984Spacecraft Prognoz 9Orbit high apogee, 700,000 kmProduct Resolution 5.5◦

Product Accuracy 0.6 mK%Data Availability lambda.gsfc.nasa.gov/product


6.3.4.3 Wilkinson Microwave Anisotropy Probe (WMAP)WMAP is a follow-on mission to the COBE mission to COBE [map.gsfc.nasa.gov].

It uses additional microwave channels and provides finer angular resolution of theCBR anisotropy, to 13 arcminutes (Spergel et al., 2003).

Table 6.18: Sensor characteristics for WMAP.Sensor WMAPSensor Type differential radiometerSensor Scan precessing helical scanPolarization multiCenter Frequency 22,30, 40, 60, & 90 GHzFundamental Measurement Differential brightness temperature (dTb)Coverage full-skyMission Length Oct. 2001 to presentSpacecraft WMAPOrbit Earth-Lunar L2Key Products* Differential Tb mapProduct Resolution 0.3◦ × 0.3◦

Product Accuracy 35 µK%Data Availability lambda.gsfc.nasa.gov/product/map

* one of several mission products


6.4 Active Systems (Radars)Unlike radiometers which are receive-only devices, radar systems include a trans-mitter. Scene properties are inferred from the echo of the transmitted signal ratherthan the radiometric emissions of the surface. Indeed, the radiometric scene emis-sion is “noise” to the radar which is interested in the echo of the transmitted signal.

6.4.1 IntroductionRadar systems can be divided into two types: real-aperture and synthetic aper-ture radar (SAR) systems. In the latter, resolution is improved by using coherenttime/Doppler processing, multiple transmit pulses, and a moving platform to, ineffect, synthesize a larger antenna than the physical antenna. This requires signifi-cant computation, high bandwidth, and high transmit power, resulting in greater ex-pense. Typically, synthetic aperture radars are more difficult to calibrate (Freeman,1992) than real aperture radars. SAR systems can provide the highest resolutionmicrowave measurements.

6.4.2 Radar FundamentalsRadar is an acronym for radio detection and ranging that was coined during its earlydevelopment. Early radars were used for detection of ships and aircraft. Soonit was noticed that radar signals provide information about the environment andradar-based environmental remote sensing was born.

Radars operate by transmitting a radio signal toward the target or scene ofinterest and measuring the properties of the returned echo. Unlike radiometers oroptical sensors, radar signals are coherent, that is, frequency and phase of the signalare preserved. This property can be exploited to yield more information about thetarget than is possible with incoherent sensors.

Most radars are pulsed, that is, short pulses of radio signal are individuallytransmitted and received. The transmitted pulse can be an unmodulated signalknown as continuous wave (CW) [sometimes, as interrupted continuous wave (ICW)]or modulated to permit improved range resolution. A common modulation schemeis linear frequency modulation (LFM), sometimes referred to as “chirping”. Fig-ure 6.9 illustrates a system block diagram of a typical radar.

The return echo from the scene consists of attenuated, time-delayed, frequency-shifted copies of the transmitted signal. The time delay arises due to finite speedof light and the range (sometimes referred to as the slant range) between the radarand the scene. Relative motion between the radar and scene give rise to a frequencyshift due to the Doppler effect. Attenuation results from the distance and the radar

6.4. ACTIVE SYSTEMS (RADARS) 39

T/R Switch

AntennaFrequencySynthesizer Transmitter

Receiver

SignalProcessor

Transmitted signal

Echo (backscattered signal)

Surface

Figure 6.9: General block digram of a typical spaceborne radar system.

scattering properties of the scene. The radar can thus observe and measure (1)distance via signal time-of-flight, (2) speed via the Doppler effect, and (3) surfacescattering. The coherent nature of the signal can lead to self interference fromdifferent parts of the illuminated scene.

Some remote sensing radars use all three of these fundamental observationswhile others use only one. For example, altimeters primarily observe time-of-flightwhile scatterometers focus on scattering. Altimeters and scatterometers are typi-cally real-aperture systems, meaning that their spatial resolution is determined pri-marily by the antenna illumination footprint whereas SARs exploit time-of-flightand Doppler to provide higher resolution measurements of the radar scattering.

6.4.3 Real-aperture RadarsReal-aperture remote sensing radars include altimeters, scatterometers, and weatherradars. A brief introduction to the operation and theory of these sensors is providedin the following. Side-looking airborne radars (SLARs) are considered later alongwith SARs.


6.4.3.1 Radar AltimetryAltimeters are standard equipment on aircraft and are used for measuring the

height of the aircraft over the ground. In this application, they are primarily usedfor navigation. Altimeters used for remote sensing have been primarily used overthe ocean and the polar regions of the earth. Here we focus on these applications.

Altimeters are nadir looking instruments. An altimeter transmits a short, gen-erally modulated, pulse toward the surface (see Fig. 6.10). By measuring the timeof flight of the return echo, and by knowing the speed of light in the transmissionmedium, the distance can be determined. The most sophisticated altimeters ob-serve the power versus time profile and use the general measurement geometry tomake more precise distance measurements (see Fig. 6.11). With precise knowledgeof the orbit, the height of the altimeter over the surface can be determined, enablingmapping of the surface topography. Over the ocean the surface topography can berelated to subsurface topography and ocean currents. Variations in sea surface to-pography provide information on large-scale weather and climate such as El Nino(Fu et al., 1994).

Sea Surface

Ocean

Sea Floor

Height

Reference

Elipsoid

Orbit

Altimeter

Sea Surface

Figure 6.10: Spaceborne altimeter measurement digram. Precise measurements ofthe height coupled with precise orbit prediction enable very precise estimation ofthe surface height relative to the reference ellipsoid.

The shape of the altimeter return pulse is related to the significant wave height,which can thus be inferred from the power versus time measurements. Significantwave height is a measure of large waves on the surface. The received echo power


Amplitude

Transmit pulse

Pulse time

of flight Echo amplitude

Time

Slope of

leading edge

Figure 6.11: Pulse shape information for a spaceborne altimeter measurement overthe ocean. From the time of flight measurement, the altimeter height is computed.The peak amplitude of the echo is related to the near-surface wind speed. The slopeof the leading edge is related to the significant wave height.

can be converted to scattering measurements for use in inferring the near-surfacewind speed over the ocean (Fu et al., 1994, 2001). The altimeter height measure-ments can also be used to measure glacial ice topography (Martin et al., 1983).

6.4.3.2 Radar ScatterometryA radar scatterometer is designed to determine the normalized radar cross sec-

tion (σo) of the surface. In primary application these σo measurements over theocean are used to estimate the near-surface vector wind.

The scatterometer does not directly measure σo, but instead, measures thebackscattered power of a transmitted pulse. Due to thermal noise in the receiver,radiometric noise and speckle, the power measurements are corrupted by noise. Aseparate measurement of the noise-only power is made and subtracted from the sig-nal+noise measurement to yield a backscatter power “signal” measurement PS . σo

is computed from the signal power measurement using the radar equation (Ulabyet al., 1981),

σo =(4π)3R4L

PtG2λ2APS = XPS (6.13)

where R is the slant range to the surface, Pt is the transmitted power, PS is thereceived backscattered power, L represents known system losses, G is antennagain, A is the resolution element area, and λ is the wavelength of the transmittedradiation. R, G, and A depend on the measurement geometry of each resolution


element (see Fig. 6.9). The measurement signal to noise ratio (SNR) depends onthe surface σo but may vary from -20 to 50 dB.

The scatterometer measurement includes errors due to the uncertainty in thegeometric parameters (i.e., in X) as well as noise due to the finite times used toestimate PS (Fischer , 1972). A common model for the actual value of σo, σo(k),observed by the scatterometer is

σo(k) = σom(k)[1 + Kp(k)ν(k)] (6.14)

where σom(k) is the actual σo of the surface for the kth measurement, Kp(k) is the

normalized standard deviation of the measurement errors, and ν(k) is a normally-distributed Gaussian random variable. Kp is sometimes known as the scatterometer“radiometric accuracy”. The multiplication in Eq. (6.14) arises, in part, as a resultof speckle noise, an inherent limitation in coherent radars. The Kp in Eq. 6.14 doesnot include the effects of “model” errors, i.e., variations in the observed σo due tovery small-scale spatial variations in σo related to inhomgeneities in the observedsurface. This effectively increases the measurement Kp (uncertainty).

The primary application of spaceborne scatterometry has been measuring near-surface winds over the ocean. By combining σo measurements from different az-imuth angles, the near-surface wind vector over the ocean’s surface can be de-termined using a geophysical model function (GMF) which relates wind and σo

(Naderi et al., 1991). An illustration of the GMF at Ku-band is shown in Fig. 6.12.More recently, scatterometer data has been used for studies of land, vegetation

and polar ice (Long et al., 2001). Because of the sensitivity of the radar backscatterto liquid water, scatterometer data has been particularly useful in polar climatestudies, see for example, (Figa-Saldana et al., 2002; Long and Drinkwater, 1999).

SASS, NSCAT and ESCAT were fan-beam scatterometers while SeaWinds em-ploys a dual rotating pencil-beam antenna (see Fig. 6.14). In a fan-beam scatterom-eter, along-track resolution results from a combination of a narrow antenna patternand the timing of transmit pulses integrated into a single measurement cell. Cross-track resolution is obtained either by range gate filtering (ESCAT) or by Dopplerfiltering (SASS and NSCAT), and the narrow beam pattern.

Range gating is used to achieve along-beam resolution for ESCAT while Dopplerfiltering is used for SASS and NSCAT. For ESCAT, short, high-power transmitpulses are issued. By selectively integrating the return signal over a narrow timewindow, only the return echo from a small along-beam resolution element willbe included in the integrated power measurement (see Fig. 6.15). For SASS andNSCAT, Doppler filters define along-beam resolution. SASS used fixed filterswhile NSCAT used variable center frequency filters to maintain measurements at afixed cross-track position (Naderi et al., 1991). The SASS and NSCAT Doppler fil-


10

5

0

-5

-10

-15

-20

-25

-30

-35

-400 40 80 120 160 200 240 280 320 360

Wind Speed (m/s)510152030

Relative Wind Direction (deg)

σ (d

B)o

10

0

-10

-20

-30

-40

-50

1 10 100

σ (d

B)o

Wind Speed (m/s)

01020ο

30405060

IncidenceAngle

(a) (b)

οοοο

οο

Figure 6.12: The geophysical model function relating near-surface winds and radarbackscatter at Ku-band. (a) Radar backscatter versus wind speed at different inci-dence angles and a fixed wind direction. (b) Radar backscatter versus relative winddirection (the direction difference between the wind and the azimuthal radar lookdirection) for several wind speeds at a fixed incidence angle. By measuring thebackscatter at a point from several different azimuth angles, the vector wind (speedand direction) can be estimated.

tering approaches preclude the use of an antenna at precisely 90◦ to the along-trackvelocity vector as used for ESCAT.

Measurement timing is used to achieve along-track resolution. Ideally the cen-ters of resolution cells are spaced approximately 25 km for NSCAT and 50 km forSASS apart in the along-track direction choosing the pulse repetition frequency andnumber of pulses integrated so that the spacecraft moves 25 km [50 km for SASS]between measurements. A series of pulses are integrated into one σo measurement.

To contrast and compare the Doppler and ranging approaches used by the ERSscatterometer and SASS/NSCAT, consider Fig. 6.16 which compares the SASS/NSCATantenna illumination pattern of a forward looking fan-beam antenna super-imposedon isoDoppler lines on the Earth’s surface and range-compression resolution basedon iso-range lines. In Doppler filtering-based resolution a long (1.5 ms) CW pulseis transmitted, resulting in a Doppler/time history plot as is illustrated in Fig. 6.17.The return echo has a Doppler shift related to the along-beam distance. A band-pass filter corresponds to a resolution element defined by the Doppler frequenciesand the narrow beamwidth. We note that this approach cannot be used when theantenna is aligned perpendicular to the flight track where the Doppler shift is min-imized.

The alternate approach to achieving along-beam resolution, is to use range-


Figure 6.13: False-color image of the Earth generated from Ku-band scatterometerdata. Colors over land and ice covered regions correspond to the intensity of theradar backscatter. Over the ocean, colors correspond to the wind speed estimatedfrom the scatterometer measurements with yellow being high speeds and blue lowspeeds. The white streamlines show the wind direction.


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Power

Time

Range Gate

Figure 6.15: Diagram of the power versus time for an echo signal illustrating rangegating.


Along-Track

Cross-Track

Iso-Doppler

Lines

Iso-range

Lines

Along-track

Cross-track

Figure 6.16: Methods for along-beam resolution for a long, thin antenna pattern.(left) Iso-Doppler filtering. (right) Range gating or range compression.

Doppler History

DopplerShift

Time

ResolutionCell

Cross-TrackDistance

Time

Resolutioncell

Range Gate

Figure 6.17: Diagram of the time/Doppler history of (left) a long-pulse echo signaland (right) a short-pulse echo signal for a forward-facting fan-beam antenna. Thegray area indicates the region of Doppler shift and time delay present in the echo.


gating as also illustrated in Fig. 6.16. In this case, a short pulse is used. The timeof flight of the pulse corresponds to the range and by selecting a narrow range oftime-of-flights (a narrow range gate), an along-beam resolution element is defined.Figure 6.16 illustrates the relationship between the time-of-flight and along-beamdistance. We note that this approach can be used even for antennas aligned perpen-dicular to the flight track. Also, longer pulses can be used with range compressionto achieve finer time resolution (see later discussion under SAR range resolution).

Both the Doppler and isorange approaches define, in effect, cross-track resolu-tion elements which cover a wide swath to the side of the flight path (see Fig. 6.18).By selecting the appropriate range gate timing and/or Doppler filter bandwidth,cross-track aligned resolution elements are defined for the forward and aft-facingantennas that enable multi-azimuth observation of the same location on the Earthas the spacecraft passes over the site (Naderi et al., 1991).

Swathgap

Outer swathincidence angle

Inner swathincidence angle

Sensor

Figure 6.18: Side-looking radar swath for a wideswath radar. Note the variation inincidence angles over the swath. The azimuth angle may be tilted forward, aft orbe orthogonal to the flight path. It is generally orthogonal for SAR systems.

Note that, unlike a radiometer and the helically scanning SeaWinds scatterome-ter, a fan-beam scatterometer measures σo at a variety of different incidence angles,see Fig. 6.18. Since the target response is different at different incidence angles,one must be extremely careful when combining measurements taken at different in-cidence angles. A solution to this problem is to use a model for the target response(such as the geophysical model function used for wind retrieval over the ocean).Over most natural surfaces in the incidence angle range 20◦ ≤ θ ≤ 60◦, corre-sponding to the range of scatterometer measurements, a well known approximate


model expresses σo (in dB) as a linear function of incidence angle, i.e.

σodB(θ) = A + B(θ − 40◦) (6.15)

where A is the 40◦ incidence angle-normalized σo and B is the dependence of σo

on the incidence angle θ. The A and B coefficients are functions of the geophysicalproperties of the surface. Note that 40◦ is the approximate center incidence angleover the swath and is a convenient angle for making comparative analyses1 .

6.4.4 Side-Looking Airborne RadarSide-looking airborne radar (SLAR) is an imaging radar based on real-aperturetechniques (Ulaby et al., 1981). Using a narrow fan-beam antenna and range com-pression, radar backscatter images can be generated. Range resolution is definedby the pulse compress, while azimuth resolution is dictated by the azimuth beamwidth of the antenna illumination pattern. To further improve the azimuth resolu-tion, synthetic aperture techniques are used. Due to the low resolution of SLARwhen used from space, few SLAR systems have been orbited. The Russian Okeanand Almaz sensors are the exceptions.

6.4.5 Synthetic Aperture RadarSynthetic Aperture Radar (SAR) has its origins in SLAR but has improved azimuthresolution due to the use of Doppler information in the return echo. This techniquewas originally called Doppler beam sharpening, but is known today as syntheticaperture radar. SAR theory is covered in extensive detail in a number of sources,e.g. (Curlander and Mcdonough, 1991; Elachi, 1988; Henderson and Lewis, 1998;Franschetti and Lanari, 1999). The general goal of SAR is to provide radar imagesof the surface (Leberl, 1990; Oliver and Quegan, 1990).

The most common form of SAR used in remote sensing is strip-map SAR,which generates a long strip-map image. A brief introduction to strip-map SAR isgiven in the following. Spotlight mode SAR differs in that the antenna is pointed,or steered, toward the target area as the platform moves past (Jakowatz et al., 1996).ScanSAR is closely related to strip-map SAR, but rapidly switches between multi-ple antenna beams to increase the swath width.

As a radar moves past the target transmitting pulses, each of the echoes fromthe target have a different frequency shift due to the relative velocity of the plat-form and the surface. This can be explained as due to the Doppler effect (hence, the

1While not applicable for all targets, over the Amazon Rainforest (which exhibits high volumescattering) a “gamma” normalization may be used with some success, i.e., γ(k) = σo(k)/ cos θ(k)where θ(k) is the incidence angle of the kth measurement of σ◦. Over the incidence angle range[20◦, 60◦], γ(k) is approximately constant, i.e., not a function of incidence angle.


original description, Doppler sharpening), but is better explained as being due tothe change in phase of the signal due to the changing range to the target. Range res-olution is typically obtained using pulse compression and matched filtering. Usingmatched filtering of the pulses in the along-track direction to select the phase his-tory of the target generates azimuth resolution, effectively narrowing the azimuthbeamwidth. This azimuth matched filter processing is equivalent to synthesis ofan array antenna in the motion direction where each element of the synthetic arraycorresponds to the actual antenna at the locations of the received pulses.

An approximation of the azimuth resolution for synthetic aperture radar can beestimated using the idea of a synthetic array antenna. As shown in (Elachi, 1988),a radar with an azimuth antenna of length Laz moving at a constant velocity, thelength of the synthetic array is equivalent to the size of the beam footprint on theground. The size of the synthetic array is given as

Ls =2λh

Laz

(6.16)

where Laz is the azimuth size of the antenna and h is the height of the platform.The new 3dB azimuth beamwidth of the synthesized array is

θSAR ≈λ

Ls

=Ls

2h. (6.17)

The corresponding azimuth synthesized resolution is given by

δSAR ≈ hθSAR =Ls

2. (6.18)

From this we note that the effective resolution of a SAR image is independent ofthe platform height.

While synthetic aperture radar provides a large improvement in azimuth res-olution over real aperture radar, it requires coherent radar hardware to preservesignal phase, storage of the received echos, and complicated computer processingto produce an image.

SARs have been used in a variety of applications, from vegetation monitoringto ocean sensing (Elachi, 1988; Elachi et al., 1984; Jackson and Apel, 2004).

6.4.5.1 Interferometric SARUnlike optical sensors, SAR is a coherent (phase preserving) technique which

measures range. Preservation of the signal phase can be exploited to provide topo-graphic information using interferometric techniques (Hanssen, 2001; Hendersonand Lewis, 1998; Madsen et al., 1993). An interferometric SAR uses two antennasat different positions to infer scene topography. While most interferometric SAR


systems do this simultaneously, interferometric processing of multiple spaceborneSAR passes over a scene has been demonstrated (see, for example, (Henderson andLewis, 1998)).

As seen from Fig. 6.19, when two SAR range observations, ra and rb, from dif-ferent positions are available, basic trigonometry enables estimation of the heightof the target relative to a reference.

ra

rb

_`

ab

cd

θ

α

H

h

channel B

channel A

b

Figure 6.19: Interferometric SAR geometry model.

The height of the target can be found using the law of cosines (Franschetti andLanari, 1999),

r2

b = r2

a + b2 − 2bra sin(θ − α). (6.19)

The target height, h, is then found by the equation

h = H − ra cos(θ). (6.20)

Typically, one of the ranges, the length of the baseline, and the height, H , areknown exactly. This implies that the accuracy of the target height estimate, h, isdependent upon the measurement of the second range, ra, or upon the change inrange, ∆r, which is defined as

∆r = rb − ra. (6.21)

Graham (Franschetti and Lanari, 1999) first proposed the idea of using the phasedifference from two SAR images, slightly offset, to determine the value of ∆r. The


idea is that if one of the received SAR images has a signal given by

γ1 = A1 exp (−j4π

λra) (6.22)

and a second SAR image, which is slightly offset from the first, has a signal of

γ2 = A2 exp (−j4π

λrb), (6.23)

then the phase difference

∆φ = arg [(γ1)(γ2)∗] =

4π

λ∆r (6.24)

enables estimation of the range difference, ∆r. The accuracy of the estimate isrelated to the size of the wavelength and the size of the baseline, b. Two collo-cated single-look complex (SLC) SAR images, one from each channel, are used tomake a phase difference image known as an interferogram. The phase differencescontained in interferograms are initially wrapped between 0 and 2π and requiretwo-dimensional phase unwrapping in order to find the ∆r estimate (Curlanderand Mcdonough, 1991). Once the unwrapped phase is computed, the height iscomputed, the phase that would be due to a “flat-earth” is computed and removedfrom the interferogram. When a high quality digital elevation map (DEM) is avail-able, the “flat-earth” phase may be replaced with the DEM phase. The topographyrelative to a “flat-earth” reference is then computed from the residual phase (Mad-sen et al., 1993).

Interferomtery has found many applications including high-resolution digitalelevation map (DEM) creation (Madsen et al., 1993) and differential images usingmultiple interferograms (Reigber and Scheiber, 2003). Along-track interferometrycan be used to extract current velocity on the ocean’s surface, as well as groundmoving targets.

6.4.5.2 Polarimetric SARPolarimetric SAR is a relatively recent development (Ulaby et al., 1990). In

polarimetric SAR, multiple combinations of transmit and receive polarization (andthe phase relationship between the polarizations) are used to create SAR imagery.The additional images provide more information about the surface and in particularprovide additional information about the number of bounces in the scattered signal(Elachi and Ulaby, 2000). This improves surface characterization.

6.4.6 Airborne Radar SystemsAircraft were the first platforms widely used for microwave remote sensing. Al-timeters are standard equipment on aircraft and are used for measuring the height


of the aircraft over the ground. Altimeters used for remote sensing have been pri-marily used over the ocean and the polar regions of the earth.

A number of scatterometers and SARs have been operated from aircraft bothfor research and surveillance. The list is too long to consider here. SAR andside-looking real-aperture radars are commonly used on military aircraft; however,these are not generally available as mapping data sets.

NASA’s Jet Propulsion Laboratory has developed a number of airborne SARsystems including AIRSAR, POLSAR, TOPSAR, and others which have beenextensively used in studies (Rodriguez and Martin, 1991; Zebker and Goldstein,1986). Many other organizations have developed airborne SAR systems, includingERIM, Donier, etc.

6.4.7 Spaceborne Radar SystemsWith their larger power requirements to support an active transmitter, fewer earth-observing radar remote sensing systems have flown than radiometer systems. How-ever, many spaceborne radar systems have flown and are currently operating. Thenext section provides a brief summary of significant microwave radar remote sens-ing systems.

6.5 Spaceborne Radar SystemsThe goal of this section is to summarize basic sensor information about importantEarth-observing radar which have operated aboard spacecraft. Historical sensorsare considered first, followed by contemporary (recent and currently operating sen-sors) and then planned sensors. Within each category, the main classes of activesensors (altimeters, scatterometers, SARs, and weather radars) are separately con-sidered in sequence.

6.5.1 Historical Radar SensorsThe landmark Seasat satellite launched in 1978 incorporated each of the major mi-crowave sensor classes, including a radiometer (SMMR), an altimeter, a scatterom-eter (SASS) and a SAR. Some of these (notably the active sensors) represent thefirst of this class of sensor to fly in space (Fu and Holt, 1982; Gloersen and Barath,1977; Grantham et al., 1977; Njoku et al., 1980). In this section we consider theseand other historical active sensors.

6.5.1.1 Historic Altimeters

6.5. SPACEBORNE RADAR SYSTEMS 53

Spaceborne altimetry has a long history of ocean measurements. Selected his-toric altimeters are described in this section.

6.5.1.1.1 GEOS-3 - The GEOS-3 altimeter flew in 1974 (Martin and Kolenkiewicz,1981). This research mission was designed to determine the feasibility of space-borne altimeters to map sea surface topography and map the geod.

Table 6.19: Sensor characteristics for the GEOS-3 Altimeter.Sensor GEOS-3 AltimeterSensor Type altimeterSensor Scan nadirPolarization N/ACenter Frequency Ku-band 13.9 GHzOperating Bandwidth 80 MHzPulse Length 12 or 200 nsPRF 100 HzFundamental Measurement time-of-flight (s)Fundamental Resolution pulse-limited, footprintCoverage global oceans, 60N to 63SFootprint Shape circularIncidence Angle 0◦

Mission Length 1975 - 1978Spacecraft GEOS-3Orbit 818 k by 858 km

115◦ inclination angleKey Products sea-level height

significant wave heightwind speed

Product Resolution 50 cmProduct Accuracy topography: 50 cm

significant wave height: 25%Data Availability podaac.jpl.nasa.gov


6.5.1.1.2 SeaSat Radar Altimeter (ALT) - Part of the landmark Seasat-1 mission,the Seasat radar altimeter (ALT) was designed to measure wave height and thedistance between the spacecraft and the ocean surface. When combined with ac-curate orbit determination, precise maps of sea surface topography were produced.Greenland ice sheet topography was also estimated (Martin et al., 1983). Over theocean, the height accuracy was estimated to be within ±10 cm (Townsend, 1980).ALT operated for 3 months in 1978 and set the stage for follow-on missions.

Table 6.20: Sensor characteristics for the Seasat Radar Altimeter.Sensor Seasat Radar AltimeterSensor Type altimeterSensor Scan nadirPolarization N/ACenter Frequency Ku-band 13.56 GHzOperating Bandwidth 312.5 kHzTransmit power 2 kW peakFundamental Measurement time-of-flight (s)

radar backscatter (σo)Fundamental Resolution pulse-limited, footprintCoverage global oceans, 60N to 63SSwath Width 2.4 to 12 kmFootprint Shape circular ringFootprint Size 2.4 to 12 kmIncidence Angle variable, 0◦

Mission Length 7 Jul. 1978 - 10 Oct. 1978Spacecraft Seasat-1Orbit 790 km sun-synchronous

108◦ inclination anglenear-circular

Key Products sea-level heightsignificant wave heightwind speed


significant wave height: 0.5 m or 10%σo: 1 dB

Product Range 0 to 20 mData Availability podaac.jpl.nasa.gov


6.5.1.1.3 TOPEX/Poseiden - The joint U.S.-French TOPEX/Poseidon mission waslaunched on 10 Aug. 1992 (Fu et al., 1994). The spacecraft carried a dual-frequency (C- and Ku-band) altimeter, a second single frequency (Ku-band) al-timeter, and a nadir looking microwave radiometer to measure water vapor (Fu etal., 1994). Designed to measure sea-level accuracy to better than 4.2 cm, it pro-vides coverages from 66◦ N to 66◦ S. The orbit was carefully choosen to minimizethe effects of tidal aliasing so as to minimize measurement error.

Table 6.21: Sensor characteristics for the Topex/Poseidon Altimeter.Sensor Topex/Poseidon AltimetersSensor Type altimeterSensor Scan nadirPolarization N/ACenter Frequency C-band: 5.3 GHz, Ku-band: 13.6 GHzOperating Bandwidth 320 MHzFundamental Measurement time-of-flight (s)

radar backscatter (σo)Fundamental Resolution pulse-limited footprintSwath Width (nadir viewing)Footprint Shape circularIncidence Angle 0◦

Mission Length 1996-2006Spacecraft TOPEXOrbit 1336 km non-sun-synchronous

circular66◦ inclination angle10 day repeat cycle



significant wave height: 13 cmσo: 1 dB

Data Availability podaac.jpl.nasa.gov


6.5.1.1.4 Geosat - Geosat was a unique gravity-gradient stablized spacecraft de-signed to carry a radar altimeter (MacArthur et al., 1971) for the U.S. NAVY. TheGeosat altimeter is essentially a copy of the Seasat altimeter. Geosat operated forthree years.

Table 6.22: Sensor characteristics for the Geosat Altimeter.Sensor Geosat AltimeterSensor Type altimeterSensor Scan nadirPolarization N/ACenter Frequency 13.50 GHzOperating Bandwidth 320 MHzTransmit power 20 WAntenna beamwidth 2◦

Fundamental Measurement time-of-flight (s)radar backscatter (σo)

Fundamental Resolution pulse-limited footprintFootprint Shape circularIncidence Angle 0◦

Mission Length 1986-1990Spacecraft GeosatOrbit 760 km by 817 km

108.05◦ inclination angle17 day repeat cycle



significant wave height: 0.5 m or 10%σo: 1 dB

Data Availability www.nodc.noaa.govwww.nsidc.org


6.5.1.1.5 ERS-1/2 Altimeter - The European Earth Resources Satellite (ERS) -1and -2 included an altimeter, though the data utility is somewhat limited by the factthat the sun-synchronous orbit results in aliasing of the ocean tidal signature, whichhas the effect of reducing the accuracy of the height measurements (Sandwell,1990).

Table 6.23: Sensor characteristics for the ERS-1 Altimeter.Sensor ERS-1 AltimeterSensor Type altimeterSensor Scan nadirPolarization N/ACenter Frequency Ku-band 13.8 GHzOperating Bandwidth 400 MHzFundamental Measurement time-of-flight (s)

radar backscatter (σo)Fundamental Resolution pulse limited, footprintFootprint Shape circularIncidence Angle variable, 0◦

Mission Length ERS-1: 1991-1996ERS-2: 1995-

Spacecraft ERS-1/ERS-2Orbit 780 km sun-synchronous

98.5◦ inclination angleKey Products sea-level height

significant wave heightwind speed


significant wave height: 13 cmData Availability podaac.jpl.nasa.gov


6.5.1.2 Historic ScatterometersScatterometry began with an experiment on a manned mission, which demon-

strated its utility for land and oceand observation. Since then, several types ofscatterometers have flown in space. Selected historic scatterometers are describedin this section.

6.5.1.2.1 Skylab S-193 - The Skylab S-193 altimeter/scatterometer was the firstspaceborne radar scatterometer and was flown as a demonstration experiment (Young,1976). It was used to study the Ku-band radar signature of the land and ocean. S-193 measurements, coupled with aircraft observations, suggested that near-surfacewinds could be measured from space and lead to the development of all subsequentwind scatterometers.

Table 6.24: Sensor characteristics for the Skylab S-193 Scatterometer.Sensor Skylab S-193 ScattterometerSensor Type scatterometerSensor Scan pencil-beamPolarization HH, VVCenter Frequency 14.6 GHzTransmit power 100 WFundamental Measurement radar backscatter (σo)Fundamental Resolution footprintCoverage spotSwath Width N/AFootprint Shape ellipticalFootprint Size incidence angle dependentIncidence Angle variableSpacecraft SkylabKey Products radar backscatterData Availability (unknown)


6.5.1.2.2 Seasat Scatterometer (SASS) - SASS was the first spaceborne wind scat-terometer (Grantham et al., 1977; Grantham, 1982; Johnson et al., 1980). It oper-ated for 3 months in 1978, collecting Ku-band σo data at nominally 50 km resolu-tion over a variable and irregular swath. SASS used 4 dual-pol fan-beam antennasto provide along-track resolution and fixed frequency Doppler filters to providealong-track resolution. Designed for wind observations, SASS could be operatedin a variety of modes to experiment with the optimal polarization mix. Unfortu-nately, the fixed frequency Doppler filters employed limited the fore/aft coregistra-tion of the backscatter measurements. SASS provided an extensive global data setof Ku-band backscatter measurements (Kennett and Li, 1989; Long and Drinkwa-ter, 1994; Long et al., 2001) and lead to a number of follow-on missions, includingNSCAT (Naderi et al., 1991) and SeaWinds. In addition to pre-launch calibration,post-launch calibration was conducted using distributed earth targets such as theAmazon forest (Birrer et al., 1982; Long and Skouson, 1996). The SASS antennameasurement geometry is illustrated in Fig. 6.20. Along-beam resolution is pro-vided by Doppler filtering, see Fig 6.21.

Antenna 3

Left SideSwath

Right SideSwath

Ground TrackAntenna 4 Antenna 1

Antenna 2

700 km 700 km

135°

45° 45°

135°

400 km

InnerSwath

~500 km~500 km

Full OverlapSwaths

Figure 6.20: Illustration of the SASS measurement geometry showing the 3dB an-tenna illumination footprints. Doppler filtering resolves the footprints into smallersegments known as “cells”. Grey region indicates portions of swath for which theforward and aft-looking measurements overlap.


DopplerFilterBandwidth

AntennaNarrowbeamBeamwidth

Measurement Area

SpacecaftGround Track

AntennaNarrowbeamGain Pattern

Doppler FilterResponse

IsoDoppler Line

IsoDoppler Line

Figure 6.21: Illustration of the SASS and NSCAT resolution element (cell) mea-surement geometry which is based on Doppler filtering and the antenna illumina-tion pattern.


Table 6.25: Sensor characteristics for SASS.Sensor Seasat Scatterometer (SASS)Sensor Type fixed Doppler, scatterometerSensor Scan fan-beam, 4 beams, dual-sidedPolarization VV, HHCenter Frequency 14.6 GHzOperating Bandwidth CWTransmit power 110 WPulse Length 5 msFundamental Measurement radar backscatter (σo)Fundamental Resolution footprintRadiometric Accuracy (Kp) 0.05Radiometric Stability (±σo) 0.15 dBCoverage globalSwath Width 500 kmFootprint Shape fan-beamFootprint Size variableIncidence Angle variable, 0◦–70◦

Mission Length 1978Spacecraft SeasatOrbit 805 km sun-synchronousKey Products ocean winds

radar backscatterProduct Resolution 50 kmProduct Accuracy 2 m/s, 20◦

Product Range 0-35 m/sData Availability winds and backscatter : podaac.jpl.nasa.gov

backscatter images : www.scp.byu.edu


6.5.1.2.3 NASA Scatterometer (NSCAT) - Designed to measure near-surface windsover the ocean, NSCAT was a follow-on to SASS (Naderi et al., 1991) and useda digital Doppler processor rather than fixed analog filters (Long et al., 1988).The digital processor ensured a fixed-width swath with better fore/aft measure-ment coregistration (Naderi et al., 1991). NSCAT included an additional antennaover SASS and provided 25 km resolution measurements on a regular 25 km grid.It operated for nine months before the spacecraft power system failed in June 1996.Like SASS, NSCAT post-launch calibration was conducted using distributed earthtargets such as the Amazon forest (Tsai et al., 1999; Zec et al., 1999). NSCATbackscatter data has been widely used in studies of land and ice, e.g., (Long etal., 2001; Long and Drinkwater, 1999). The NSCAT measurement geometry isillustrated in Figs. 6.22 and 6.21.

Right SideSwath

Satellite Ground Track

Antenna 6Beam 6V

600 km

45 ° °

° 135

MonitorCells

°

° 65 45

115135°

Antenna 5Beam 5H,5V

Antenna 4Beam 4V

Antenna 3Beam 3V

Antenna 2Beam 2H,2V

Antenna 1Beam 1V

Left SideSwath

600 km175

175 km km

Figure 6.22: Illustration of the NSCAT measurement geometry showing the 3dBantenna illumination footprints. Doppler filtering resolves the footprints intosmaller segments known as “cells”.


Table 6.26: Sensor characteristics for NSCAT.Sensor NASA Scatterometer (NSCAT)Sensor Type variable Doppler scatterometerSensor Scan fan-beam, 6 beams, dual-sidedPolarization VV, HHCenter Frequency 13.995 GHzOperating Bandwidth ICWTransmit power 110 WPulse Length 5 msPRF 62 HzFundamental Measurement radar backscatter (σo)Fundamental Resolution footprintRadiometric Accuracy (Kp) 0.02-0.07Radiometric Stability (±σo) 0.1 dBCoverage global, 2 daysSwath Width two 600 km swaths with 700 km gapFootprint Shape fan-beamFootprint Size 5 × 950 kmIncidence Angle variable, 17◦–60◦

Mission Length 1996-97Spacecraft ADEOS-I (Midori-I)Orbit 955 km sun-synchronous, 10 a.m.Key Products ocean winds

radar backscatterProduct Resolution 25 km, 50 kmProduct Accuracy 2 m/s, 20◦




6.5.1.2.4 ERS AMI Scatterometer Mode - The European Remote Sensing (ERS)ERS-1 and ERS-2 Advanced Microwave Instrument (AMI) in scatterometer modeuses multiple fan-beam antennas to make σo measurements at different azimuthangles over a wide swath (Attema, 1991). Each antenna produces an instantaneousfootprint several hundred kilometers long and a few kilometers wide. The antennaswere arranged at three azimuth angles and sweep out a nominally 500 km wideswath. This swath is resolved into smaller cells using pulse timing and range gatefiltering as described below. Nominally 50 km resolution σo measurements arereported on 25 km centers for each antenna. The along-track timing is complicatedby the synthetic aperture radar (SAR) wave mode which time shares the transmitter.Unfortunately, scatterometer data cannot be collected during SAR operations.

Swath

Ground Track

Antenna 1

Antenna 2

Antenna 3

500 km

45°

90°

135°

~250 km

Figure 6.23: Illustration of the ERS-1/2 AMI scatterometer mode (ESCAT) mea-surement geometry showing the 3dB antenna illumination footprints. Range gatefiltering resolves the footprints into smaller areas.

The ERS-1/2 AMI digitizes the received echos and telemeters the data to theground for further processing. On the ground, the pulses corresponding to eachalong-track cell are integrated. Range gating is separately applied to each echo toachieve cross-track resolution and the power computed. The pulses correspondingto each along-track cell are then integrated into a single “50 km” resolution mea-surement. A 25 km spacing grid of nominally 50 km σo measurements are obtained


from each antenna. While not precisely defined in the available literature, a spatialsmoothing filter is applied when integrating the pulses, For each grid element thereare three measurements of σo, one from each antenna beam. In many land andice applications these may be combined; however, care must be used for surfacesexhibiting azimuthal variation in σo (notably the ocean) (Early and Long, 1997).The ERS scatterometer measurement geometry is illustrated in Figs. 6.23 and 6.24.Due to apacecraft attitude system failure, the ERS-2 scatterometer ceased globalcoverage in 2001, though limited coverage is available through 2008.

AntennaNarrowbeamBeamwidth

MeasurementArea

SpacecraftGroundTrack

AntennaNarrowbeamGain Pattern

Range FilterResponse

Cross-track

Along-track

Iso-RangeLines

50 km

100 km

3 dBPoint

a)

b)

50 km

50 k

m

Figure 6.24: (top) ERS-1/2 scatterometer mode measurement geometry. Resolu-tion is based on range-filtering. (bottom) ERS-1/2 scatterometer mode σo geome-try. Multiple pulse measurements are averaged into (a) 50 km measurements on a25 km grid weight with (b) a Hamming window.


Table 6.27: Sensor characteristics for the ERS-1/2 AMI Scatterometer Mode.Sensor ERS-1/2 AMI Scattterometer ModeSensor Type range-gate, scatterometerSensor Scan fan-beam, 3-beams, single sidedPolarization VVCenter Frequency 5.3 GHzPulse Length 70-135 µsPRF ∼ 127 HzOperating Bandwidth ∼ 15 kHzTransmit power 5 kWFundamental Measurement radar backscatter (σo)Fundamental Resolution footprintRadiometric Accuracy (Kp) 0.05Radiometric Stability (±σo) 0.57 dBCoverage globalSwath Width 500 kmFootprint Shape fan-beam, averagedFootprint Size 5 × 750 kmIncidence Angle variable, 18◦–69◦


Spacecraft ERSOrbit 780 km sun-synchronous

98.5◦ inclination angleKey Products ocean winds

radar backscatterProduct Resolution 50 km, 25 km postingProduct Accuracy 4-24 m/s: 2 m/s or 10%, 20◦

Product Range 0-35 m/sData Availability winds and backscatter : www.ifremer.org



6.5.1.3 Historic SARsGiven their large power requirements and high data rates, a surprising number

of SAR systems have flown in space with several on the U.S. Space Shuttle pro-gram. Selected historic SAR sensors are described in this section. Because of thecomplexity of the sensors and their coverage swaths, only very brief summariesof the sensors are provided. Readers seeking additonal information are referred tocited references and data distribution sites.

6.5.1.3.1 Seasat SAR - The Seasat SAR provides the first synoptic, high-resolutionSAR images of the Earth’s surface (Fu and Holt, 1982; Jordan, 1980). It operatedfor 99 days in 1978. Data was collected only when the spacecraft was in view of aground station which limited coverage to areas around Goldstone, California; Fair-banks, Alaska; Merritt Island, Florida; Shoe Cove, Newfoundland, Canada; andOakhanger, United Kingdom. Originally, SAR processing was done optically, buteventually the entire archive was digitally processed. Approximately 500 passes of5 min each were collected.

Table 6.28: Sensor characteristics for the Seasat SAR.Sensor Seasat SARSensor Type SARSensor Scan fan-beam, SARPolarization HHCenter Frequency 1.275 GHzPulse Length 33.4 µsPRF 1463-1640Operating Bandwidth 19 MHzTransmit power 1 kWFundamental Measurement radar backscatter (σo)Coverage limited to near ground stationsSwath Width 100 kmIncidence Angle variable, 20◦–26◦

Mission Length 99 days, 1978Spacecraft Seasat-AOrbit 800 km circular sun-synchronous

108◦ inclination angleKey Products radar backscatter imagesProduct Resolution sim25 mData Availability southport.jpl.nasa.gov/scienceapps/seasat.html


6.5.1.3.2 Shuttle Imaging Radar A (SIR-A) - SIR-A was the first of the NASA/JPLshuttle radar series and was essentially a duplicate of the Seasat SAR. It waslaunched on Nov. 12, 1984 aboard the Space Shuttle Columbia and used a fixedlook angle. Data was collected over a several day period.

Table 6.29: Sensor characteristics for SIR-A.Sensor SIR-ASensor Type SARSensor Scan fan-beam, SARPolarization HHCenter Frequency 1.275 GHzPulse Length 30.4 µsPRF 1464-1824 HzOperating Bandwidth 6 MHzTransmit power 1 kWFundamental Measurement radar backscatter (σo)Coverage near-globalSwath Width variable, 50 kmIncidence Angle variable, 47◦–53◦

Mission Length few days in 1984Spacecraft Space Shuttle (STS-2)Orbit 222 km × 231 km near circular

38◦ inclination angle37 orbits (3 days)

Key Products radar backscatter imagesProduct Resolution 40 m × 40 mData Availability southport.jpl.nasa.gov


6.5.1.3.3 Shuttle Imaging Radar B (SIR-B) - SIR-B was the second in the NASA/JPLshuttle radar series (Cimino et al., 1986). It was launched on October 5, 1984aboard the Space Shuttle Challenger on flight 41-G into a nominally circular orbit.Like SIR-A it used a fixed look angle and and operated from the U.S. space shut-tle. However, the shuttle altitude was varied during the mission to enable multipleincidence angle observations of the same location. Data was recorded on-boardthe space shuttle using a film recorder for later processing but has not been madewidely available.

Table 6.30: Sensor characteristics for SIR-B.Sensor SIR-BSensor Type SARSensor Scan fan-beam, SARPolarization HHCenter Frequency 1.275 GHzPulse Length 30.4 µsOperating Bandwidth 12 MHzTransmit power 1.12 kWFundamental Measurement radar backscatter (σo)Coverage limited near-globalSwath Width variable, 20-40 kmIncidence Angle variable, 15◦–65◦

Mission Length few days in 1984Spacecraft Space Shuttle (STS-41G)Orbit 360, 257, 224 km circular

57circ inclination angleKey Products radar backscatter imagesProduct Resolution 20-40 m × 16-58 m


6.5.1.3.4 Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR) - SIR-C/X-SAR was a joint project of the National Aeronautics andSpace Administration (NASA), the German Space Agency (DARA) and the ItalianSpace Agency (ASI) (Evans et al., 1997; Jordan et al., 1991; Stofan et al., 1995).The multipolarization SIR-C SAR operated at L-Band and C-Band while X-SARoperated at X-band. The SIR-C/XSAR system was flow twice aboard the U.S.Space Shuttle in 1994, STA-59 9 to 20 April and STS-68 30 September to 11 Oc-tober 1994. The shuttle flew in 225 km in a circular, 57◦ inclination orbit. SIR-Cused an active phased array antenna which could be electronically steered. X-SARwas mechanically tilted. Data was recorded on-board the space shuttle for groundprocessing.

Table 6.31: Sensor characteristics for SIR-C/X-SAR.Sensor SIR-C/X-SARSensor Type SARSensor Scan fan-beam, SARPolarization SIR-C: HH, VV, VH, HV

X-SAR: VVCenter Frequency SIR-C: 1.275 and 5.17 GHz

X-SAR: 9.67 GHzPulse Length SIR-C: 33.8, 16.9, 8.5 us

X-SAR: 40 usPRF 1395-1736Operating Bandwidth 10, 20, and 40 MHzTransmit power SIR-C: 1.275 GHz: 4400 W

5.17 GHz: 1200 WX-SAR: 1400 W

Fundamental Measurement radar backscatter (σo)Radiometric Stability (±σo) SIR-C: 1.5 dB & X-SAR: 2.5 dBCoverage limited coverage, near-globalSwath Width 15 km to 90 km, variableIncidence Angle variable, 17◦–63◦

Mission Length two 11 day missions in 1994Spacecraft Space Shuttle STS-59 & STS-68Orbit nominally 225 km circularKey Products radar backscatter images

some interferometric imagesProduct Resolution ∼ 30 mData Availability southport.jpl.nasa.gov

www.asf.alaska.edu


6.5.1.3.5 Shuttle Radar Topography Mission (SRTM) - Using modified SIR-C/X-SAR hardware to support interferometric operation, SRTM became the first space-borne single-pass interferometer. It obtained global elevation data to create themost complete (80% of the Earth’s surface) high-resolution digital topographicdatabase of Earth ever made (Farr et al., 2007; Moreira et al., 1995; Coltelli et al.,1996). SRTM flew onboard the Space Shuttle Endeavour during an 11-day missionin February of 2000. The data set has become an important one in earth studies.

A second antenna system was added to the SIR-C/X-SAR system to enablesingle pass cross-track interferometric measurements. The “outboard” antennasextended 60 m from the main antenna in the shuttle bay in order to form the inter-ferometric SAR baseline for the C- and X-band channels. The L-band channel ofSIR-C was not used for SRTM. SRTM extracted topographic information from theSAR data on a 30 m grid. The estimated vertical accuracy is 16 m.


Table 6.32: Sensor characteristics for SRTM.Sensor SRTM (modified SIR-C/X-SAR)Sensor Type interferometric SARSensor Scan dual antenna fan-beam, SARPolarization SIR-C: HH, VV, VH, HV

X-SAR: VVCenter Frequency SIR-C: 5.17 GHz

X-SAR: 9.67 GHzPulse Length SIR-C: 33.8, 16.9, 8.5 us

X-SAR: 40 usPRF 1395-1736Operating Bandwidth 10, 20, and 40 MHzTransmit power SIR-C 5.17 GHz: 1200 W

X-SAR: 1400 WFundamental Measurement radar backscatter (σo)

inteferometric path lengthCoverage 60N to 56SSwath Width 50 kmIncidence Angle variable, 17◦–63◦

Mission Length one 11 day mission in Feb. 2000Spacecraft Space Shuttle (STS-99)Orbit 233 km circular

57◦ inclination angleKey Products topographyProduct Resolution 30 mProduct Accuracy 16 m absoluteData Availability srtm.usgs.gov


6.5.1.3.6 Japanese Earth Remote Sensing (JERS-1) SAR - The Japanese NationalSpace Agency (NASDA) launched JERS-1 (also known as Fuyo-1) into a 570 km(354 mi) orbit on February 11, 1992 (Nemoto et al., 1991; Shimada, 2006). Thissatellite included a seven band optical sensor and an L-Band SAR which operatedat HH polarization. The SAR had a fixed look angle view between 32◦ and 38circ

with a swath width of 75 km and a mean resolution of 18 m.

Table 6.33: Sensor characteristics for the JERS-1 SAR.Sensor JERS-1 SARSensor Type SARSensor Scan fan-beam, SARPolarization HHCenter Frequency 1.275 GHzPRF 1505-1606 HzOperating Bandwidth 15 MHzTransmit power 1100-1500 WFundamental Measurement radar backscatter (σo)Fundamental Resolution footprintCoverage globalSwath Width 75 kmIncidence Angle 32◦–38◦

Mission Length Feb. 1992 – Oct. 1998Spacecraft JERS-1Orbit 570 km sun-synchronous

38.5◦ inclination angleKey Products radar backscatter imagesProduct Resolution 1 18 m, 100 mData Availability www.asf.alaska.edu

eoportal.org

6.5.2 Contemporary Spaceborne Radar SensorsCurrently (early 2008), representatives of each of the major class of active mi-crowave remote sensors are currently flying in earth orbit. The following subsec-tions briefly describe these.


6.5.2.1 Contemporary Altimeters6.5.2.1.1 JASON-1 and JASON-2 - A follow-on to Topex/Poseidon, Jason-1

was launched on 7 Dec. 2001. It carries a dual-frequency (C-band and Ku-band) al-timeter (Poseidon-2) and a nadir-looking radiometer, the NASA Jason MicrowaveRadiometer (JMR), which is designed to measure water vapor along the path of thealtimeter signal in order to correct the signal path measurement (Jason, 2004). Thefollow-on Jason-2 is scheduled for launch in June 2008.

Table 6.34: Sensor characteristics for the Jason-1/2 Poseidon-2 Altimeter.Sensor (Jason-1/2) Poseidon-2 AltimeterSensor Type altimeterSensor Scan nadirPolarization N/ACenter Frequency C- (5.3 GHz) and Ku-band (13.6 GHz)Operating Bandwidth 320 MHzTransmit power 7WPulse Length 105 usPRF 1700 HzFundamental Measurement time-of-flight (s)

radar backscatter (σo)Fundamental Resolution pulse limited, footprintCoverage 95% of ice-free oceans in 10 days

66N to 66SFootprint Shape circularIncidence Angle variable, 0◦

Mission Length Jan. 2002 - presentSpacecraft Jason-1 & Jason-2Orbit 1336 km non-sun-synchronous

66◦ inclination angle10 day repeat cycle


Data Availability podaac.jpl.nasa.govaviso.cnes.fr


Table 6.35: Sensor characteristics for JMR.Sensor Jason-1 Microwave Radiometer (JMR)Sensor Type RadiometerSensor Scan nadirPolarization N/ACenter Frequency 18.7, 23.8, 34 GHzFootprint Shape circularIncidence Angle 0◦

Mission Length Jan. 2002 - currentSpacecraft Jason-1Orbit 1336 km non-sun-synchronous


Fundamental Measurement Brightness temperature (Tb)Key Products atmospheric liquid water

water vaporData Availability podaac.jpl.nasa.gov

earth.esa.int


6.5.2.2 Contemporary ScatterometersCurrently (early 2008), two scatterometers are operating in Earth orbit, rep-

resenting the two types of scatterometers: fan-beam and pencil-beam. Primarilydesigned to measure winds over the ocean, they are also used for land, ice, andvegetation studies.

6.5.2.2.1 QuikSCAT/SeaWinds - Scanning pencil-beam scatterometers such as Sea-Winds offer an alternative design to the fan-beam NSCAT and ERS-1/2 scatterom-eters. This design concept results in smaller, lighter instruments with simpler field-of-view requirements while maintaining a low data rate. As a result, a pencil-beamscatterometer can be more easily accommodated on spacecraft than a fan-beam. Akey difference between fan-beam and pencil-beam scatterometers is measurementdwell time. Fan-beam scatterometers permit longer dwell times, albeit with a re-duced SNR, compared to a pencil-beam scatterometer system (Spencer et al., 2000,2003). Though designed as a scatterometer, radiometric measurements useful forrain-flagging can be derived from the data (Jones et al., 2000). The SeaWindsscanning geometry is illustrated in Fig. 6.25.

Two identical copies of the SeaWinds instrument have flown: the first, whichwas actually the engineering prototype, was launched in 1999 aboard the QuikSCATmission and is still in operation as of early 2008. The second, flew aboard the ill-fated ADEOS-II (Midori-2) mission for 10 months in 2003.

Cross-track

18 rpm

Inner beam

Outer beam

Nadir track

900 km700 km

SeaWinds

sweet zone

sweet zone

Figure 6.25: Illustration of the SeaWinds measurement geometry showing the 3dBantenna illumination footprints and scanning geometry employed.


Table 6.36: Sensor characteristics for SeaWinds/QuikSCAT.Sensor SeaWinds Scatterometer

QuikSCAT ScatterometerSensor Type pencil-beam scatterometerSensor Scan dual-scanning pencil-beamPolarization outer beam: VV

inner beam: HHCenter Frequency 13.6 GHzOperating Bandwidth 250 kHzTransmit power 110 WPulse Length 1.5 msPRF 192 Hz (two beams)Fundamental Measurement radar backscatter (σo)Fundamental Resolution footprintRadiometric Accuracy (Kp) 0.02-0.07Radiometric Stability (±σo) 0.1 dBCoverage global, 2 daysSwath Width 1800 kmFootprint Shape ellipticalFootprint Size 20×30 km , 25×35 kmIncidence Angle variable, 17◦–60◦

Mission Length QuikSCAT: 1999 to presentSeaWinds/ADEOS-II: 2003

Spacecraft QuikSCAT, ADEOS-II (Midori-II)Orbit 795 km sun-synchronousKey Products ocean winds

radar backscatterProduct Resolution 25 kmProduct Accuracy 2 m/s, 20◦


winds : www.remss.combackscatter images : www.scp.byu.eduwinds : www.remss.com


6.6.2.2.2 Advanced Scatterometer (ASCAT) - The advanced scatterometer (AS-CAT) is a follow-on for the ERS-1/2 scatterometer series (Figa-Saldana et al.,2002). The first ASCAT was launched in 2006 aboard the meteorological (MetOp-A) platform. Two additional launches are planned in later years. Generally similarto the European Remote Sensing (ERS) ERS-1 and ERS-2 Advanced MicrowaveInstrument (AMI) in scatterometer mode, ASCAT uses multiple fan-beam antennasto make σo measurements at different azimuth angles over a wide swath. However,ASCAT has a dual-sided swath to provide additional coverage (see Fig. 6.26) andthe resolution is improved to 25 km on a 12.5 km grid. The range of incidenceangles relative to ESCAT is increased to improve wind measurement accuracy,though this increases the sensitivity to rain.

Ground Track

Right Swath

550 km

45°

90°

135°

~360 km~360 km

Left Swath

550 km

90°

45°

135°

~360 km

Figure 6.26: Illustration of the ASCAT measurement swath geomtetry. Range gatefiltering resolves the footprints into smaller areas.


Table 6.37: Sensor characteristics for ASCAT.Sensor ASCATSensor Type range-gate, scatterometerSensor Scan fan-beam, 6-beams, dual sidedPolarization VVCenter Frequency 5.255 GHzPulse Length 8.03 ms & 10.1 msPRF 29.12 HzOperating Bandwidth 401 kHz & 242 kHzTransmit power 125 WFundamental Measurement radar backscatter (σo)Fundamental Resolution footprintRadiometric Accuracy (Kp) 3.3% to 9.9%Radiometric Stability (±σo) 0.46 dBCoverage globalSwath Width dual 550 km swathsFootprint Shape fan-beam, averagedIncidence Angle variable, 25◦–65◦

Mission Length 5 yearsSpacecraft MetOpOrbit 822 km sun-synchronous, 21:30 ascending node

98.7◦ inclination angle29 day and 5 day repeat cycles

Key Products ocean windsradar backscatterice productssoil moisture

Product Resolution 50 km & 25 kmposted on 25 km and 12.5 km grids

Product Accuracy 2 m/s, 20◦

Product Range 0-35 m/sData Availability www.meteorologie.eu.org/safo


6.5.2.3 Contemporary SARsAs of early 2008, several earth-observing SARs are operating in Earth orbit,

providing high resolution radar imagery at L-, C-, and X-band. Selected sensorsare considered below.

6.5.2.3.1 ERS-1/2 AMI SAR - The ERS-1 and ERS-2 Advanced Microwave In-strument (AMI) in SAR mode collects C-band SAR image data in a number ofmodes (Attema, 1991). In addition to conventional high resolution SAR images,the AMI SAR can be operated in “wave mode” in which lower resolution imagesare collected over a wide swath in order to estimate ocean wave spectra. Power andthermal considerations limit the time the instrument can be operated each orbit toabout 12 minutes per orbit.


Table 6.38: Sensor characteristics for ERS-1/2 AMI SAR Mode.Sensor ERS-1/2 AMI SAR ModeSensor Type SAR, wave mode SARSensor Scan SAR, wide-beam SARPolarization VVCenter Frequency 5.3 GHzOperating Bandwidth 15.55 MHzTransmit power 5 kWPulse Length 37 µsPRF 1640-1720 HzFundamental Measurement radar backscatter (σo)Fundamental Resolution 16 mRadiometric Accuracy (Kp) <2.5 dB at ±σo = -18 dBRadiometric Stability (±σo) 0.95 dBCoverage limitedSwath Width 20 km, (image mode) 80 km, (wave mode) 500 kmFootprint Shape push-broomIncidence Angle variable, 18◦–20◦, 18◦–50◦


Spacecraft ERS-1, ERS-2Orbit 785 km sun-synchronous (100 min period)

98.5◦ inclination angle35 day repeat cycle (3 and 168 also used)

Key Products radar backscatterocean wave spectrasea ice

Product Resolution 16 m, 30m, 100 mData Availability www.esa.int

www.asf.alaska.edu


6.5.2.3.2 RADARSAT - The RADARSAT-1 satellite was launched on 4 Nov. 1995.It’s primary sensor is a SAR system capable of multiple operational modes. Themain beam is steerable. RADARSAT is operated as a commercial venture by theCanadian government. Unlike many other sensors, data must be purchased. Costof data is often a limiting factor in studies using this sensor. The RADARSAT SARhas seven beam modes with different resolutions and coverage2 .

Table 6.39: RADARSAT-1 SAR modes.Mode Resolution Incidence angles Swath widthStandard 25 m 20-49◦ 100 kmWide 25 m 20-45◦ 150 kmExtended Low 25 m 10-20◦ 75 kmExtended High 25 m 50-60◦ 75 kmFine 9 m 35-49◦ 50 kmScanSAR Wide 100 m 20-50◦ 500 km, 440 kmScanSAR Narrow 50 m 20-46◦, 32-46◦ 300 km

2http://www.ccrs.nrcan.gc.ca/ccrs/data/satsens/radarsat/specs/radspec e.html


Table 6.40: Sensor characteristics for RADARSAT-1 SAR.Sensor RADARSAT-1 SARSensor Type SAR, wave mode SARSensor Scan SAR, scanSARPolarization VVCenter Frequency 5.3 GHzFundamental Measurement radar backscatter (σo)Fundamental Resolution 16 mRadiometric Stability (±σo) 0.3 dBCoverage global, 4-5 daysSwath Width 50–500 kmFootprint Shape push-broomIncidence Angle variable, 10◦–59◦

Mission Length Nov. 1995 to presentSpacecraft RADARSAT-1Orbit 798 km, sun-synchronous, 18:00 ascending node

98.6◦ inclination, 100.7 min periodKey Products radar backscatter images

sea iceProduct Resolution 8–100 mData Availability www.rsi.ca

www.asf.alaska.edu


6.5.2.3.3 ALOS PALSAR - The Phased Array type L-band Synthetic ApertureRadar (PALSAR) is an improved version of the JERS-1 SAR and was developedas a joint project between the Japanese space agency (JAXA) and the Japan Re-sources Observation System Organization (JAROS) (Rozenqvist et al., 2007; Wak-abwayashi et al., 1998). PALSAR is part of the Advanced Land Observing Satellite(ALOS), also known as Daichi, launched on 24 Jan. 2006. PALSAR is capableof wideswath (scanSAR), as well as polarimetric operation. ALOS also carriesthe Advanced Visible and Near Infrared Radiometer type 2 (AVNIR-2) and thePanchromatic Remote-Sensing Instrument for Stereo Mapping (PRISM).

Table 6.41: Sensor characteristics for PALSAR.Sensor ALOS PALSARSensor Type SARSensor Scan fan-beam, SARPolarization HH, HV, VH, VVCenter Frequency 1.270 GHzPRF 1505-1606 HzOperating Bandwidth 14 or 28 MHzTransmit power 2000 WFundamental Measurement radar backscatter (σo)Fundamental Resolution footprintRadiometric Accuracy (Kp) 1 dBRadiometric Stability (±σo) 1.5 dBCoverage globalSwath Width Fine: 30-70 km

ScanSAR: 240-350 kmIncidence Angle 9.9◦–50.8◦

Mission Length Jan. 2006 to presentSpacecraft ALOSOrbit 692 km sun-synchronous

98.16◦ inclination angle46 day repeat cycle

Key Products radar backscatter imagesProduct Resolution 10-157 m, depending on modeData Availability www.eorc.jaxa.jp

www.palsar.ersdac.or.jpenvisat.esa.intwww.asf.alaska.eduwww.alos-restec.jp


6.5.2.3.4 EnviSAT Advanced SAR (ASAR) - The European Space Agency (ESA)launched the Envisat in March 2002. It includes an Advanced SAR (ASAR) sen-sor. Operating at C-band, ASAR continues the SAR image dataset started withERS-1/2 but includes dual polarization capability. EnviSAT also carries the RA-2radiometer. Envisat is operated as a commercial venture and data must be pur-chased (seehttp://earth.esrin.esa.it/pub/ESA DOC/ENVISAT/ENVI87.pdf).

The Envisat ASAR operates in imaging mode similar to the ERS-1/2 AMISAR with wide swath mode (400 km swath, 150 m resolution, single polarization)and an alternating polarization (56-100 km swath, 25 m resolution, selectable HHpolarization) mode. It employs seven selectable beams with an active phased arrayantenna.


Table 6.42: Sensor characteristics for Envisat ASAR.Sensor Envisat ASARSensor Type SAR, wave mode SARSensor Scan SAR, wide-beam SARPolarization VV, HH, VH, HVCenter Frequency 5.331 GHzOperating Bandwidth 50 MHzTransmit power 5 kWFundamental Measurement radar backscatter (σo)Fundamental Resolution 16 mRadiometric Accuracy (NSE) ∼ -25 dB

worst-case better than -20 dBRadiometric Resolution 1.5-3.5 dBRadiometric Stability (±σo) 0.3 dBCoverage globalSwath Width 20 km, 100 km, 400 kmFootprint Shape push-broomFootprint Size 30 m × 30 m, 150 m × 150 m, 1 km × 1 kmIncidence Angle variable, 15◦ −−45◦

14.5◦–22.1◦ 104 km swath18.8◦–26.1◦ 103 km swath25.8◦–31.2◦ 82 km swath30.8◦–36.1◦ 89 km swath35.7◦–39.3◦ 64 km swath39.0◦–42.7◦ 71 km swath42.5◦–45.2◦ 57 km swath

Mission Length Dec. 2007 to presentSpacecraft EnvisatOrbit 800 km sun-synchronous (101 min period)


Key Products radar backscatterocean wave spectra

Product Resolution 30 m, 150 m, 1 kmData Availability envisat.esa.int


6.5.2.3.5 TerraSAR-X - TerraSAR-X is a German X-band SAR similar to X-SAR,but with additional capability including spotlight, stripmap, and scansar modes(Stangl et al., 2006). It also supports interferometric operation via a second space-craft (Krieger et al., 2007). The highly capable TerraSAR-X was launched on 15June 2007.

Table 6.43: Sensor characteristics for TerraSAR-X.Sensor TerraSAR-XSensor Type SAR, interferometric SARSensor Scan SAR, wide-beam SARCenter Frequency 9.65 GHzOperating Bandwidth 5-300 MHz, 150 MHz nominalTransmit power 2260 WPRF 3-6.5 kHzFundamental Measurement radar backscatter (σo)Fundamental Resolution spotlight: 1-2 m

stripmap: 3-6 mscanSAR: 16 m

Radiometric Accuracy (NSE) ∼ -25 dBworst-case better than -20 dB

Radiometric Resolution 1.5-3.5 dBRadiometric Stability (±σo) 0.3 dBCoverage globalSwath Width spotlight 10 km

stripmap: 30 kmscanSAR: 100 km

Footprint Shape push-broomIncidence Angle variable, 15◦ −−60◦

Mission Length June 2007 to presentSpacecraft TerraSAR-XOrbit 514.8 km sun-synchronous

18:00 ascending node97.44◦ inclination angle11 day repeat cycle

Key Products radar backscatterDEM

Data Availability www.dlr.de


6.5.2.3.6 Okean - The Russian Okean satellite series included a number of mi-crowave radar remote sensors. For example, the Okean-O satellite had a side-looking real aperture radar (RLSBO-D) with a swath width of 700 km and a resolu-tion of 1.5 km × 2 km and a Delta-2 scanning radiometer with a swath width of 900km and a resolution of 16 km× 21 km [http://www.astronautix.com/craft/okeano.htm]It is not clear what data is available from these sensors.

6.5.2.3.7 Almaz - Since the Russian Almaz are a series of real-aperture radar im-agers, they are not technically SARs, but are included in this section since they aredesigned for radar imaging. The first Almaz was flown in 1987. Another Almaz3

was launched in 1991. The SAR used two 0.5 x 15 m slotted waveguide antennas.

Table 6.44: Sensor characteristics for Almaz.Sensor AlmazSensor Type real-aperture imagerSensor Scan fan-beam, range-compressedCenter Frequency 3 GHzTransmit power 190 kW (peak)Fundamental Measurement radar backscatter (σo)Fundamental Resolution 15-30 mCoverage limitedSwath Width 20-45 kmIncidence Angle variableMission Length 18 monthsSpacecraft Almaz-1Key Products radar backscatter imagesProduct Resolution 15-30 m

6.5.2.4 Contemporary Weather RadarsGround-based weather radars have been extensively deployed to study rain-

fall. However, only spaceborne weather radars are considered here. One of themost important rain radars that has flown to date is the TRMM precipitation radar.Cloudsat carried a related radar designed to measure clouds.

3http://www.globalsecurity.org/space/world/russia/almaz.htm


6.5.2.4.1 Tropical Rain Measuring Mission (TRMM) Precipitation Radar (PR) -The TRMM PR was orbited in 1995 (Awaka et al., 1997; Meneghini et al., 2000).It uses electronic switching of 128 antenna elements in a 2 m by 2 m antennato provide 49 fixed beams in a cross-track push-broom configuration over a 220km wide swath (see Fig. 6.27. The PR employs pulse compression to provide 250m vertical resolution measurements of atmospheric scattering and attenuation fromwhich rain rate estimates are derived. Surface backscatter data is also used in oceanwind speed, land cover, and vegetation studies. TRMM has far exceeded its designlife and has provided an extensive data set over the tropics (latitude less than ±30◦). TRMM PR data is augmented by TMI radiometer data.

NadirTrack

220 km

Figure 6.27: An illustration of the TRMMPR measurement geometry. Switchedantenna elements form 48 different spot beams across the swath that are used insequence to scan across the swath. Each beam operates at a different incidenceangle.


Table 6.45: Sensor characteristics for TRMM PR.Sensor TRMM PRSensor Type Precipitation radarSensor Scan electronically switched pencil-beam

in push broom configurationPolarization HCenter Frequency 13.8 GHzOperating Bandwidth 6 MHzTransmit power 500 WPulse Length 1.67 µsPRF 2776 HzVertical Resolution 250 mCoverage 40S to 40NSwath Width 220 kmFootprint Shape ellipticalFootprint Size 4.3×4.3 km to 5.0×5.8 kmBeamwidth ∼ 71◦

Incidence Angle ±17◦

Mission Length Dec. 1995 to presentSpacecraft TRMMOrbit 350 km high circular, semi-equatorial

402.5 km high after Aug. 2001Fundamental Measurement Brightness temperature (Tb)Key Products vertical and horizontal distribution

of preciptationProduct Resolution 4.3 km (nadir)

5 km (swath edge)Product Accuracy min Z: 20.8 dBZProduct Range 0-15 kmData Availability www-dsdis.gsfc.nasa.gov


6.5.2.4.2 CloudSat Cloud Profiling Radar (CPR) - The CPR was orbited aspart of the CloudSat mission in 2007. The CPR is a nadir-looking 94 GHz radarthat is designed to measure the backscatter as a function of range (height) in clouds(Stephens et al., 2002). Its spatial coverage is limited to very close to the nadirtrack,

Table 6.46: Sensor characteristics for CPR.Sensor CloudSat CPRSensor Type cloud profiling radarSensor Scan nadir-looking pencil-beamPolarization circularCenter Frequency 94 GHzOperating Bandwidth 6 MHzPulse Length 3.3 µsPRF 4300 HzRadiometric Accuracy -25 dBZ(min detectable Z)

Radiometric Stability (±σo) 1.5 dBVertical Resolution 500 mVertical Range 0-25 kmSwath Width 1.4 kmFootprint Shape circularFootprint Resolution 1.4 km × 2.5 kmMission Length April 2006 to presentSpacecraft CloudsatOrbit 705 km altitude, circularFundamental Measurement reflectivity versus rangeKey Products vertical distribution of water dropletsProduct Accuracy min Z -30.8 dBZProduct Range 0-15 kmData Availability www.cira.colostate.edu


6.5.3 Future Radar SensorsMany new spaceborne active microwave sensors are in various planning stages inthe U.S., Japan, Europe, China, and India. Many of these are extensions of existingmissions. Many new missions are also planned. It is not possible to do anythingmore than describe a few of them.

6.5.3.1 RADARSAT2RADARSAT-2 is a follow-on to RADARSAT-1 but with additional capabilities,

including high resolution, multipolarization and both left- and right-looking imag-ing. Like RADARSAT-1, RADARSAT-2 will be operated as a commercial ventureby the Canadian government. Unlike many other sensors, data must be purchased.RADARSAT-2 was successfully launched on 14 Dec. 2007. Like RADARSAT-1,RADARSAT-2 SAR has eleven operational beam modes4.

Table 6.47: RADARSAR-T SAR modes and resolutions.RADARSAT-2 Mode Resolution Incidence angles Swath widthStandard 25 m 20-49◦ 100 kmWide 25 m 20-45◦ 150 kmLow Incidence 35 m 20-23◦ 170 kmHigh Incidence 24 m 50-60◦ 70 kmFine 10 m 37-49◦ 50 kmScanSAR Wide 100 m 20-49◦ 500 kmScanSAR Narrow 50 m 20-46◦ 300 kmStandard Quad Polarization 27 m 20-41◦ 25 kmFine Quad Polarization 10 m 20-41◦ 25 kmMulti-Look Fine 10 m 30-50◦ 50 kmUltra-fine 3 m 30-40◦ 20 km

4http://www.ccrs.nrcan.gc.ca/ccrs/data/satsens/radarsat/rsat2/mission e.html


Table 6.48: Sensor characteristics for the RADARSAT-2 SAR.Sensor RADARSAT-2 SARSensor Type SAR, wave mode SARSensor Scan SAR, scanSARPolarization HH, HV, VH, VVCenter Frequency 5.405 GHzOperating Bandwidth 100 MHzPRF variableFundamental Measurement radar backscatter (σo)Fundamental Resolution mode dependentRadiometric Stability (±σo) 0.3 dBCoverage global, equator: 2-3 days

north of 70N: dailySwath Width 50–500 kmFootprint Shape push-broomIncidence Angle variable, 10◦–59◦

Mission Length 7 yearsSpacecraft RADARSAT-2Orbit 798 km, sun-synchronous,214 day repeat

98.6◦ inclination angle100.7 min period

Key Products radar backscatter imagessea ice

Product Resolution 3–100 m

6.5.3.2 Aquarius - The planned Aquarius mission includes both a radiometer anda scatterometer and is designed to make precise global observations of sea surfacesalinity from space. The Aquarius sensor suite is described in Section 6.3.3.

6.5.4 Extra-Terrestrial Radar Sensors

A variety of earth-based microwave sensors have been used in astronomical ob-servations. Both active and passive sensing have been used. Passive radio tele-scopes are extensively employed for the study of extra-solar objects (Burke andGraham-Smith, 2002; Rohlfs and Wilson, 1996; Verschuur et al., 1987); however,ground-based radars have been used to study the moon and other planets (Evansand Hagfors, 1968; Pettengill, 1970; Plaut and Arvidson, 1992). Spacraft-basedimaging radar systems have been deployed to study cloud-covered Venus, and morerecently, Titan. Here, brief descriptions of several of these sensors are provided.


The dense cloud clover of Venus precludes conventional optical sensing of thesurface, necessitating the use of radar to probe beneath the thick atmosphere. Anumber of U.S. and Russian spacecraft have carried radar systems designed toimage through the atmosphere. Both the U.S. Magellan mission (Pettengill et al.,1991) and the Russian Venera 15 and 16 probes (Bogomolov et al., 1985) carriedradar sensors which combined SAR and altimeter functions.

6.5.4.1 Pioneer Venus Orbiter RadarThe Pioneer Venus Orbiter Radar is described in (Cuttin et al., 1984; Dallas,

1987).

6.5.4.2 Venera SARIn 1983 the two Soviet Venera 15 and 16 spacecraft began their missions to

orbit Venus. Operating in tandem, the two identical spacecraft carried SAR systemsdesigned to image the surface (Bogomolov et al., 1985; Ivanov, 1990). The SARantenna was 6 m by 1.4 m while the altimeter antenna was 1 m in diameter. Aseparate 2.6 m antenna was used for communication to the earth.

Table 6.49: Sensor characteristics for Venera SAR.Sensor Venera 15/16 RadarsSensor Type SARSensor Scan fan-beam SARCenter Frequency 3.75 GHzPulse Length 1.54 µsPRF burst, 127 pulses/burstTransmit power 8 WFundamental Measurement radar backscatterFundamental Resolution SARCoverage 25% of Venus surface south of 30NSwath Width 130 kmMission Length 8 months beginning in 1984Spacecraft Venera 15 & 16Orbit elliptical, apsis of 1000 kmKey Products radar backscatter imageProduct Resolution 1-2 kmData Availability nssdc.gsfc.nasa.gov


6.5.4.3 MagellanThe Magellan mission was launched on 4 May 1989. It went into an orbit

around Venus on 10 Aug. 1990 and carried a radiometer, an altimeter (ALTA),and a synthetic aperture radar (Dallas and Nickle, 1987; Pettengill et al., 1991;Rokey, 1993; Saunders et al., 1992, 1990). To save cost and weight, the main3 m antenna was shared between the SAR sensor and communications. A 189minute elliptical orbit was designed to bring the spacecraft close to the surfaceto improve the imaging signal-to-noise ratio at the orbit perigee. Data collectedduring the perigee was recorded during collection and broadcast to earth duringthe apogee. To account for the variable altitude, a complicated burst-mode transmittiming was used for SAR imaging with radiometer and alimemter measurementswere interleaved with the SAR measurements. The received data was compressedfor transmission to Earth. At the end of the mission the spacecraft was slowed andallowed to enter the atmosphere.


Table 6.50: Sensor characteristics for the Magellan SAR.Sensor Magellan SARSensor Type SARSensor Scan fan-beam SARPolarization HHCenter Frequency 2.385 GHzPulse Length 26.5 µsPRF burst, 4.4-5.8 kHzOperating Bandwidth 2.26 MHzTransmit power 325 WFundamental Measurement radar backscatterFundamental Resolution SARRadiometric Accuracy (Kp)Radiometric Stability (±σo)Coverage 90% of Venus surface south of 30NSwath Width 25 kmFootprint Shape circularFootprint Size 25 kmIncidence Angle variableMission Length 10 Aug. 1990 – 11 Oct. 1994Spacecraft MagellanOrbit elliptical, 189 min periodKey Products radar backscatter imageProduct Resolution 150 m SAR, 30 m altimeterData Availability www.lpi.usra.edu/library/RPIF

pds.jpl.nasa.gov


6.5.4.4 Cassini - The Cassini radar instrument shares the main radio dish usedfor communication between the Cassini spacecraft and Earth and is capable ofradiometer, altimeter, scatterometer and SAR measurements, all operating at thesame frequency of 13.78 GHz. The instrument was designed to image throughTitan’s cloud cover using 5 antenna beams from its main 4 m high-gain antenna.Spatial resolution of the radiometer and altimeter is dependent on the range fromthe Cassini spacecraft and the surface, which is highly variable.

Table 6.51: Sensor characteristics for the Cassini Radar.Sensor Cassini RadarSensor Type altimeter/scatterometer/SARSensor Scan pencil-beam scanned by spacecraft motionCenter Frequency 13.78 GHzPRF variable burstOperating Bandwidth scatterometer: 117 kHz

altimeter: 4.68 MHzFundamental Measurement radar backscatter, rangeFundamental Resolution variableCoverage 20% of TitanFootprint Shape ellipticalMission Length 2004 to presentSpacecraft CassiniKey Products radar backscatter, heightProduct Resolution variable

SAR: 300 m to 2 kmaltimeter: 24-27 km horizontal

90-140 m verticalData Availability pds.jpl.nasa.gov

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